My comments are based on my experience with the horse in my avatar, that does have a fractured sacrum -- that happened after I bought him:)
1. The Seller was not truthful. SI issues just don't appear out of nowhere --- provided your massage therapists knows what he/she is talking about.
There's been an injury somewhere in the horse's life. It could have been anything from the pasture, to casting himself in a stall, to mishandling by a human, to charging backward losing his balance and falling on his butt.
IMHO, it would be best to carry the horse to a vet or hospital with the diagnostic equipment to look at the horse's skeletal structure to make an accurate diagnosis, so you know what the treatment(s) should be and what work level (if any)'the horse will be able to do in the future.
That said --- it sounds as if you did not get a PPE on this horse?
In my horse's case, the injury is a fractured sacrum (he fractured it in 2007) but his poll sometimes goes out when he is overcompensating trying to get up.
The rear hooves can also be affected in that the horse will move its back end different causing abnormal wear on one side of the hoof or the other.
I use this red light therapy pad on my horse. It is 12 years old and has more than paid for itself on a couple of horses, dogs, and my leg when I had cellulitis. https://equinelighttherapy.com
I've also got my horse on this particular Cosequin by Nutramax. It works the best of any oral supplement I have tried and it does have clinical studies behind it. https://www.chewy.com/nutramax-coseq...onic/dp/133613
Another cheap fix because I can't afford $700 for the Sportz Vibe therapy massage/heat sheet --- I gave my horse my $20 chair massage pad that I bought at WalMart for my back, and bought myself a new one. I cinch it on him with a stretchy standing wrap and leave it on each affected area 20 minutes. Not ideal but it works until I hit the lottery:)
My horse also sees a hollistic vet/chiro/acupuncturist monthly. He is 24 and his fracture is now 12 years old with a lot of arthritis build up. No injections -- the lameness vet who treated him for founder (he's also insulin resistant) nixed that in a big way.
He is not rideable but he is dealing with a serious fracture, hopefully that is not the case with your horse.
Best wishes, as SI injuries are a serious issue and to reiterate, it would be best to spend the money up front to find out precisely what the issue is.