This has been suggested more times than I can count. I personally will edit it out of titles if it's some random person with no vouches or only a couple. Otherwise, people are allowed to use almost anything they want in thread titles (aside from already established ranks such as mod, omm, highly trusted, highly respected, etc) unless they actually hold the rank - It's up to you as the person viewing their thread to make the decision of whether or not they are "trusted"
Use some common sense. Always trade safe. Always do some research on the person you're trading. May save you a buck.