Legally speaking you offered your tank for sale at a specific price and the bidder accepted. These two acts form a binding contract. So legally, yes, you have a cause of action. The real issue is not whether you have a binding contract but whether it is worth your time and effort in pursuing the claim. In my professional opinion, and yes I am an attorney, it is not worth your time, energy but more importantly money. The only vindication you have is filing a grievance against the bidder which does little.