There’s a two-part answer for this one. Yes, the seller can change their mind and refuse to sell. No one can force a seller to sign the deed, no one can force the seller to honor the contract – well, theoretically a judge or mediator can but I’ve not heard of it happening.
But that doesn’t mean there aren’t going to be ramifications if a seller simply changes his mind. Think of the expenses borne by the buyer – an appraisal, home inspections, possibly moving costs. It’s conceivable a seller could be responsible for all of these if they are in breach of the contract, if not more once damages are considered.
Bottom line is before a seller decides not to sell after an executed purchase contract is in place, that seller needs to talk to a real estate attorney post haste to find out what the ramifications of such an action might be.
Talking to your agent isn’t going to be sufficient – we’re expert in bringing buyers and sellers together and getting the contract executed; the best way to break that contract is in the purview of the attorneys.