I love how everyone is commenting like the laws are the same all around the world. Nobody except the OP can possibly know what the barn owner can or cannot do in that specific situation because we don't know where she and/or the horse is/are located. Laws vary by country, state, and city.
Where I live, a boarder must be 3 months behind on board before the stable can take the horse. After 3 months, the barn has the right to sell the horse to recoup the money, but they have to Sell it at a public auction. No private dealings are allowed. And they are only allowed to keep the past due board. If the past due board was $500 and the horse sells for $900, the barn now owes the owner $400. I believe they also have to give written notification to the owner of the horse that they will be selling the horse, but I'm not sure about that.
OP, look up the city ordinance in your area and see what the laws and regulations are. And pay your **** board.