Honestly if I feel I need my firearm to do a sale then I’m probably not doing that sale. Some of my safety rules are -
As a seller I pick the location. Usually the shopping center close to my house.
If it’s a high dollar item eg brand new iPhone, I meet at my local police station.
Already carry a counterfeit pen.
Try not to deal after dark.
Never ever trust anyone.
Collect and verify payment before buyer opens a brand new item.
For iPhones / Apple watches etc only allow buyer to login into iCloud account after payment. Once they login you can’t log them out so phone is basically theirs.
Always makes sure buyer is on camera at some point eg by entering a walmart or other store.