I spoke to my solicitor about preventing exactly this kind of thing when buying my house, as there had been a lot of interest...
Apparently unless the seller accepts some form of "consideration" (such as a deposit, or you providing some other valuable service to them) then it's effectively impossible to enforce a sale agreement as being contractually binding, even if purely for the purpose of claiming liquidated damages for losses on things like searches or surveys.
His exact words were "if you still want one, I'll happily take your money and draft it, as long as you understand you're wasting the money."; I really like that firm of solicitors, both times they've acted for me they really cut through to the heart of the issues.