Robosats uses an escrow https://learn.robosats.com/docs/escrow/ the other side has to do the payment in advance into the escrow. If they refuse to click the "I got the fiat/BCH/amazon gift card" button the SATs will be stuck in the escrow. You can then tell robosats that there is a dispute and show them the chat and proof that the other guy was dishonest. They will then unlock the escrow and send the sats to you.
Discussion
Thanks for the explsnation. This makes me much more comfortable in using them for things in the future.