You need to read and thoroughly understand this portion of IL law. If you need to, pay an attorney for an hour of his time to explain it to you in detail.
You want to follow the procedures outlined below to the letter. If you do not and you take possession of and dispose of the horse, the owners will have grounds to take legal action against you despite the fact that they owe you money.
Direct link to the Illinois Legislature's website, from where I pulled this from:
(770 ILCS 40/49) (from Ch. 82, par. 58)
Sec. 49. Stable keepers.
(a) As used in this Section:
"Boarding or stabling agreement" means any agreement, written or oral, that establishes or modifies the terms, conditions, rules, or any other provisions concerning the boarding or stabling of a horse.
"Horse owner" or "owner" means a person who owns a
horse or the person's agent, successor, or assign.
"Keeping", "boarding", or "stabling" means boarding
and training services at a stable provided at the horse owner's request including but not limited to: services under a written boarding or stabling agreement; services provided in response to a written request by the horse owner; administering the horse owner's requested supplements or medications to the horse; trailering the horse to a designated location; and assistance in conjunction with veterinary, breeding, farrier, chiropractic, dental, or other professional services for a horse.
"Last known address" means that address provided by
the horse owner in the latest boarding or stabling agreement or any other writing, or the address provided by the horse owner in a subsequent written notice of a change of address.
"Personal property left on the premises" means any
carriage, harness, tack, equipment, supplements, medications, or other personal property stored by the owner at the stable or boarding facility.
"Stable" or "horse boarding facility" means any real
property used for the purpose of stabling, boarding, or keeping a horse.
"Stable keeper" or "keeper" means the owner,
operator, lessor, or sublessor of a stable or horse boarding facility, or his, her, or its agent, or any other person or entity authorized by him, her, or it to manage the horse boarding facility or to receive rent from a horse owner under a boarding or stabling agreement.
(b) Stable keepers shall have a lien upon the horses, carriages, harness, tack and equipment, and any other personal property stored by the owner at the stable or boarding facility in the amount of the proper charges due for the keeping thereof and expenses bestowed thereon at the request of the owner, or the person having the possession thereof, plus other charges, present or future, in relation to the horse boarded, plus all fees and expenses, including legal fees, incurred by the keeper to enforce the lien, whether by public or private sale, to the extent permitted under this Section.
(c) The lien provided under this Section is a possessory lien which entitles the keeper to restrict the subject horse's movement from the keeper's facility, except for emergency veterinary care. Local law enforcement authorities notified of the keeper's right to a possessory lien on the horse shall have the authority, without a court order, to restrict the removal of the horse from the keeper's facility despite the horse owner's request or demand, until the keeper confirms to the local law enforcement authorities that the lien has been satisfied. Evidence of a possessory lien that constitutes notice to law enforcement authorities includes, but is not limited to, a written boarding agreement signed by the horse owner acknowledging that the keeper shall have an automatic lien on the horse for the amount owed or the unpaid invoices for the keeping of the horse and fees and expenses, including attorney's fees, incurred in connection with the enforcement of the lien.
(d) Enforcement of the lien must be by public sale of the horse or any personal property left on the premises, unless the written boarding agreement provides a clear authorization that the horse and any personal property left on the premises may be sold through a private sale or provides a clear authorization that the keeper may take ownership of the horse and any personal property left on the premises to satisfy the lien.
(e) Prior to enforcement of the lien through a public or private sale, the keeper must make a written demand for payment from the owner of the horse and include notice that the horse and any personal property left on the premises will be sold at a date certain not less than 30 days after service of the demand for payment if payment of all amounts due is not made by the day set for the sale.
(f) In the event of enforcement of the lien through a public sale, the sale shall be conducted by the local sheriff or other authorized individual, at which cash shall be paid by the highest bidder. The date, time, and place of the sale shall be advertised by posting a written notice of the sale at 3 of the most public places in the township or municipality where the horse is boarded or if the horse is not boarded in a township or municipality, by posting a written notice of the sale at 3 of the most public places in the county. The posting may be done at any time, but shall not be done less than 21 days before the sale. Alternatively, the sale may be advertised by placing a sale notice stating the date, time, and place of the sale for 3 consecutive weeks in a weekly or daily newspaper of general circulation distributed in the township or municipality where the horse is boarded. After the amount due for keeping the horse and any unpaid invoices and any and all fees and costs incurred in connection with the enforcement of the lien, including, but not limited to, the auctioneer and sheriff's fees, advertising costs, and any legal fees and costs are paid, the balance shall be paid to the owner. At the time that the surplus funds are received by the horse owner, the ownership papers to the horse shall be forwarded by the horse owner to the public sale buyer within 7 days after the horse owner's receipt of the surplus funds. In the event the horse owner cannot be found after reasonable attempts to locate the owner have been made, any surplus funds shall be paid to the keeper's chosen equine rescue, rehabilitation, or retirement facility or organization. At the time of the sale conducted pursuant to this Section, the sheriff or other authorized person conducting the sale shall, upon payment of the sale price, provide a public sale certificate of purchase of the horse to the highest bidder showing that the highest bidder is the new legal owner of the horse. As of the time of the delivery of the public sale certificate of purchase, the original horse owner shall have no claim of right, title, or interest in the horse.
(g) In the event of enforcement of the lien through a private sale pursuant to the terms of this Section, written notice shall be considered satisfactory under this Section when sent at least 3 weeks prior to the date of any private sale to the horse owner's last known mailing address by first class certified or registered mail or by e-mail or facsimile. At the time of the private sale conducted pursuant to the requirements of this subsection, the keeper shall, upon payment of the sale price, provide a private sale certificate for the horse to the buyer certifying that the requirements of this subsection were met and that the buyer is the legal owner of the horse. As of the time of the delivery of the private sale certificate, the original horse owner shall have no claim of right, title, or interest in the horse. The application of the sale proceeds shall be as provided in subsection (f).
(h) In the event of enforcement of the lien through the keeper obtaining ownership of the horse and any personal property left on the premises, the keeper shall provide a demand for payment to the owner pursuant to subsection (e) and shall include an appraisal of the value of the horse and any personal property left on the premises showing that the value of the horse and the personal property left on the premises is at least equal to the amount due and owing to the keeper.
(i) A lien created pursuant to this Section attaches as of the date the horse is brought to the stable and shall be superior to any other lien or security interest except for a statutory lien or security interest which is perfected through a proper filing pursuant to Article 9 of the Uniform Commercial Code prior to the beginning of the boarding.
(Source: P.A. 97-569, eff. 1-1-12.)