If someone asks for positive feedback after doing a very small trade or if they had nothing at risk, you probably should be suspicious that they're trying to farm trust. It's happened many times over the years, and I'm sure people are still trying to do it.
In my opinion there is nothing wrong with building trust among the user in this forum as long as we don't beg, feedback is only as proof that there is a smooth transaction. But of course it's true, for things that aren't high risk then I can also say it's a cheap endeavour.
After all successful trading is not only done with DT members, and this feedback shouldn't only come from them either. Any user can give his feedback on successful trades or whatever, as long as it's relevant then it's fine I guess. What's unfortunate is the abuse, so many forum members are against it.
Trust system should work as it should, even I have successfully paid off $500 loan to DarkStar_ never asked him to send me positive feedback. That is up to him, and I have also never asked anyone to include me in their trust list even if I have contributions to the local board. I know that's not how it works, so there's no need to ask for things that aren't high-risk.