You need to track the transaction ID in block explorer e.g. blockchair to see whether the transaction has been confirmed or not, it require at least 1 confirmation, you can assume it's already safe as long as you trade small amount e.g. less than 1 Bitcoin.

If the transaction is still unconfirmed, the seller can cancel the transaction.

If they only share a screenshot, don't trust since it's easily to be faked.