The only exchange between buyer and seller should be : Buyer- "I'll take it". Seller posts- SOLD subject to receipt of funds. PERIOD. Any contingency stated by buyer is just THAT, a CONTINGENT sale. Item remains for sale until contingency is agreeably removed. If another acceptable ( more -less-contingent- non contingent) offer is presented, the first deal/non deal is dead. Of course, buyer and seller CAN make ANY ( LEGAL)agreement they wish as long as BOTH are satisfied and there exists tangible consideration.
Mark
PS: Above "legal" opinion gleaned from my extended ( 1 semester" ) study of Business Law and my recollection of the Uniform Commercial Code
Mark