Before the contract is firm, if the seller's own conditions, such as finalizing a purchase of their own within a set window of 5 to 10 business days, have not been met, they may be able to walk away under the terms of that specific condition, but this depends entirely on how the contract was drafted.
Genuine exceptions are narrow: a material misrepresentation by the buyer, a failure of a condition that was actually the seller's own, or a mutual release both parties agree to. None of these are the same as simple regret.
A seller weighing this needs their own lawyer's opinion on the specific contract wording before taking any action, since the financial exposure for breaching a firm contract can be significant.
Get your hand-reviewed valuation from Joel Dyck.