Checking them out.
Bring a gun. Let the other person know that you're carrying.
Should scare off anyone who's thinking about ripping you off.
Flashing a gun is heavy-handed. Are you really prepared to shoot someone in public over some bitcoin? At least this way your lawyer can prove they stole before you went Dirty Harry.
The etiquette is that whoever has the more established reputation gets paid first, whether the buyer or seller.
Makes sense.
Anyway, this is another tool for sellers to use as it suits them.