One could get a positive trust if a DT decides to add you to their trust list or decides to write a positive feedback. This doesn't mean the person is forever trusted. Change could happen that's why you need to always check previous trade history of the person you're dealing and if you aren't sure of who you're with, you can use one of the trusted escrow on the forum.
Correct, this can happen as OP mentioned he deal with the person and now planning to make a much bigger deal, for me it's best to have reputable escrow if the person really trustable he won't be bothered to use and guide OP on how to use escrow service if he really intend to make and execute good deal for OP.