I suspect M2P and Sunnyone are right, and it may be best to just euth her. I don't know who to be angrier with here- the alleged former owners who left her to rot, or the person who wants to sell her for $500 as a broodmare.

If you do get her out of there, I'll contribute $30 toward your expenses in whatever your decision is with her (euth or rehab). Just PM me with your name and address. Money is tight right now, but I can at least do that much for her.