What if a dishonest user sends a TX with low Fee (1 Sat/vB) to a vending machine, the machine releases the product, and after some time that user performs a FullRBF changing the destination address to one of his own addresses?
Apart from what everyone already said, merchant who accept 0-conf transaction in past usually doesn't accept transaction with low fee rate anyway. By low fee rate, i mean low enough to make that transaction won't be confirmed in next few blocks based on current mempool.
- On mainnet those vending machines are not instant and need to wait up to 3 confirmations of the TX.
1 confirmation should be good enough for relative low value goods.