Trade flair is the count of confirmed trades a user has completed. It is not intended to indicate if a user is trustworthy to trade with, but rather indicated that they have followed the process a number of times.
Any 2 users may confirm a single trade once per sale thread.
Buying multiple items per transaction will only grant 1 trade confirmation.
Local trades do not grant trade flair.
When confirming a trade, either the buyer or seller must have a post to refer the trade back to One of them must be the author, and the other must have replied to the thread at some point.
If neither users are the author, that post can not be used for trade confirmation.
Make sure as a seller that you taking proper timestamps and making posts for the things you are selling. Going into another seller's thread to sell your items is not appropriate.