I admit I didn't go through all the responses, but, if you want to get the horses off property...work on selling the low priced ones first. The statute doesn't say how much you have to seel them for, just to sell them. The chances of meeting that 1200 balance is slim adn sicne the board contract isn/t in writing, and you are going into another month, tack on that extra month board..now she is up to 2400 and you could also add some late fees in there. Once the lower priced ones sell, then go for the higher value ones. You may get just enough to cover the back board and your efforts, make sure to add in time and effort to sell, ( set an hourly "wage"), and have all of the horses off property.
Now as a caveat, you need to check with a lawyer in this to make sure you CAN sell since no contract was written. The other option is small claims court.