A legally binding agreement is entered when both sides agree on the deal. This means that one party accepts an offer and the other confirms. Offers are not legally binding.
If the seller accepts an offer from a buyer, the buyer will have to confirm the deal before it is binding. If the seller sends a counter offer to the buyer and he/she accepts, the seller will have to confirm the deal before it is binding. You can always cancel a deal as long as the other party didn't confirm. Furthermore, open offers do not prevent other users from making offers as well.
Please note: Always use the offer function and don't agree on deals in the public questions and answers. If all open questions have been answered make a private offer and let your partner confirm the deal.