"Now, whats to stop the seller from sitting on the contract and not signing it for several weeks/months, delaying the sale?"
You.
Returney the contracts or we is walking away!
This is the wording used in solicitors correspondence exchanges until such time as the contracts are binding. Note the words "signed, exchanged and a full deposit is paid."
Please note that we do not have any authority either express or implied to bind our clients in any way and we must emphasise that neither this correspondence nor any negotiations had between our clients shall constitute a note or memorandum within the meaning of the Statute of Frauds until such time as both contracts are signed, exchanged and a full deposit is paid.
mf