The invoice isn;t what activates seller protection, per se. It is simply an added step. Kind of hard for the buyer to say "I ever wanted this" and then send you back an empty shoebox if you have multiple steps of him/her approving the purchase.
As far as selling in public, I have done this. Again, you can't stop a crazy person. However, some safeguards.
1.) Buy a counterfeit currency pen. The one I have also has a UV light for the hologram and cost me about $10... it will probably last a lifetime. I am a bit of a CL junkie, so I probably get more use than most, but still, it's ten bucks lol
2.) Meet the guy at a mall where there is a bank ATM. The mall I frequent has all I need: ATM, Apple store, Verizon store, AT&T store, Gamestop, etc.
3.) If it's not apparent I really like lists.