The advantage of Retoswap is that it locks up both party's Monero in a 2/3 multisig wallet (same way Bisq used to work). The seller risks 115% of the value of the transaction and the buyer risks 15% in Monero. If either tries to scam they lose everything once the arbitrator decides who's at fault.
Discussion
And how does the arbitrator figure that out?
Depends on the evidence. With registered mail, the recipient has to sign for the package and the sender has to package the cash in the presence of a postal worker and declare the value truthfully under threat of fraud charges. Then there is a legally binding chain of custody from each postal worker all the way to the recipient.
This isn't unique to Monero, Retoswap is a fork from Bisq which was the original p2p Bitcoin only exchange. Cash by mail has been a popular payment method there for years before Retoswap and it functions the same way with the security deposit + arbitrator system.
Rather just buy bitcoin from a bitcoin meetup
Yeah that works really well too, if you want to snipe a low price, Retoswap works better since you lock in the price when you accept the offer. You also gain more physical security with using the mail system.
All tradeoffs, depends on the situation.