The default contracts in virginia have a sneaky line that few ever see. It says "-OR- 1 A. (ii) Notice voiding this Contract."
Which means that all they need to do is give notice to void the contract within the review period. (usually 7 days).
You would have had to have striken that line to make the buyer be bound to the contract. That is almost never done.
But even if that was gone, they would find a way out. Consider contracts merely memos of understanding until you close.