A seller of Bitcoin can offer a contract before meeting a buyer, in which the buyer associates his intended receiving address with his GPG key (other identity / trade details can also be signed on). For trades done on camera where the buyer walks off without paying after receiving their BTC, there is time-stamped evidence that the buyer received the BTC in the blockchain and contract combo.
If a buyer wants to change their contractually-agreed receiving address upon physically meeting, that'd be a red flag.
Full details in this article.My understanding is that localbitcoins trades are done internally via escrow. The coins are released to the buyers account once the trade is complete (this is similar to sending coins to a specific address).
For trades conducted outside of localbitcoins: How do you associate a GPG key with a specific person? What would prevent you (as a seller) to simply create a GPG key, sign a message with a BTC address that you control, then send the BTC to that address? In this situation the buyer would obviously leave once s/he sees that the BTC is not xfered to the addressed specified.