As a former realtor in NY, once you accept an offer from a buyer, each party needs to sign the contract. Between the days from accepting an offer to signing the contract, you (the seller) has the legal right to continue showing your home and accept backup offers. You do need to disclose to other buyers that there is an offer and they are viewing the home as a backup. Before the contract is signed by both parties, realtors will enter your listing in the MLS as "Pending" and under "Agent Remarks" (for agent use only - not public information) agents will type in "Backup offers only"
Once the contract is signed by both parties, you need to stop all showings and remove your listing from the internet.