Can a seller withdraw the agreed offer and sell it someone else?

No the seller is legally obligated to sell to the first person that he signs a contact with. If he does this you can sue him for damages.

no it is legally binding..

in general if you have a verbal agreement, it is technically binding, but much more difficyult to enforce.  If you have a paper agreement with contradicts the verbal one, the written will always take precidence.  Mortgages and home soles are no exception.

If you have estalish all the aspects of a binding contract, primarily offer+acceptance and terms of the contract then you're bound - unless you can establish something like incapacity or duress which can overturn a contract.  That said you don't want to go there.  If you've accepted an offer then you're stuck with it.