Yes they can if it is done in accordance with the terms of the contract. If it is done in breach of the contract, the seller can actually have legal action taking against them. Not always Unless the buyer agrees, the contract remains enforceable, and a court can order the sale to proceed.
Typically a real estate contract begins with a written offer from the buyer. The offer, to be official is signed by they buyer. From there there seller may make amendments and sign and amended contract, that needs to be approved and the changes are either initialled by the buyer and the seller or a new contract containing agreed upon amendments is resigned by both parties. The signing continues until a final agreement with all agreed changes has been signed by both parties. For further information, see the related link below.
If you have both signed the contract and it is legally valid, then NO, the seller cannot change the terms of the contract or unilaterally void the contract (unless the contract states that the seller is allowed to do this). If you are in doubt, you need to talk to a lawyer ASAP.
If the seller agrees to let you out of the contract. Otherwise you are bound by what you signed.
The contract is not enforceable unless both parties signed it. If the sellers changed their mind and didn't sign then you don't have a contract.
You can't. If you signed then you have agreed to purchase the vehicle. Your only recourse is to talk to the seller and see if they will let you out of the deal. Offer to pay them to tear up the agreement.
When it has been signed by the buyer and seller.
The date the Buyer signs it.
No you cant because you have signed a contract and it means you have to deal with it till it ends.
Odds are the seller won't put any effort in keeping another buyer from getting the house from under you.
It is wise to address such matters before the contract is signed. This is dictated by the terms of the contract the parties agreed on. If you've already signed the contract then you will just have to read the contract to find out.
If a seller and a buyer have already signed a contract, then you have to sell according to the contract. If you want to sell to someone else not on the contract, then you have to get out of the first contract.
You may have a good claim. The seller is on the hook to repair whatever he agreed to fix in the contract. Your attorney should notify him via his attorney that he is in breech. If the seller fails to respond, your attorney should file a claim against him in civil court. If your attorney is reluctant, find another lawyer.