I am not a lawyer but, the buyer can file what is called a lis pendens against the property so that you cannot sell until the litigation they filed is done.

They can sue you for specific performance. If they followed what they had to do in the contract and you did not, then they should win monetary damages.

Think about the predicament you put them into. If the shoe were on the other foot......

Do get a real estate attorney if you do not want to proceed.

Definitely contact your attorney. Start with talking to your realtor as well, so he/she is aware of the situation. Potential repurcussions would be realtor suing for full commission as he/she has performed duties under the contract, and I have heard stories where buyers sue as well. Were you in under stress when selling? Did your situation change? These are things you need to discuss and get expert advice from your legal counsel.

Why did you agree to sell your house if you didn't want to?
Purchase agreements are legally binding. You could try and have a talk with them to see if they they will let you out.
They may or may not. You could be sued for performance or damages if you refuse to sell and there are no contingencies to let you out.
Best to get an attorney involved right away.
Or perhaps reconsider. There was likely a reason you wanted to sell. Has that reason changed?

That depends on the contract. You should consult your attorney. The contract will have clauses that cover possible reasons to get out of it, as well as remedies if either party violates the terms of the contract. Depending on how much the buyer has already invested in the process, financially and emotionally, would determine how likely he/she is to sue.