I had agreed to buy a business over the weekend and started filing to put in my name they just called and said they sold it for 500 more

If a signed agreement by both parties was a condition to the effectiveness of your agreement, the absence of a signed agreement defeats your claim. An attorney would need to review the email chain and other exchanges with the seller to assist you in evaluating whether to pursue this. My guess is that $500 isn't worth suing for.