Buyers can choose to use escrow for transactions, which relies on Bitcoin multisig. Instead of paying Bitcoin directly to the vendor, you can choose a moderated payment, which goes into an escrow account that can only be accessed then two out of three parties in the trade agree. The buyer and vendor are the first two parties, and they mutually agree to the third party, called a moderator. If the trade goes through normally, then buyer and seller both agree to release the funds and the vendor gets paid as normal. If something goes wrong and either party opens a dispute, then the moderator will step in and provide dispute resolution, releasing the funds to the winning party.
This moderation system means that - as long as the moderator is reliable - scams should be entirely avoidable. It’s important to pick a moderator that is trustworthy.
Apart from the standard 2-of-3 multisig used for moderated payments, OpenBazaar also uses a 1-of-2 multisig for offline ordering. When sending a direct payment to an offline vendor, a buyer can’t be certain that the vendor will ever come online, meaning their Bitcoin would be wasted: 1-of-2 multisig solves this problem by allowing either party to pull the Bitcoin out of the account at any time. If the vendor does come back online, they can accept the payment and fulfill the order. If the vendor nevers comes back online, then the buyer can just pull their money back.