I have seen numerous times where people where selling techniques or other stuff, where said good was provided for free to a DT to assess it. Can't find the thread again right now.
The most recent one which comes to mind is
this thread, which had vouches from both TMAN and actmyname.
As you say, it is a fairly common occurrence that users who are selling some digital good will offer free "vouch" copies to trusted users. It is also commonplace outside this forum that companies will give away a certain number of their product for free in return for feedback and ratings. Logically, there is not much of a difference between offering something of value for free and offering payment in return for an honest opinion, although the latter certainly
feels shadier to me. I also think asking specifically for DT members rather than just trusted members was a mistake.
Provided the review was honest and stated the user was paid for it, then although I don't like it, I can't see it being any different to handing out "vouch" copies.
Having said all that, leaving positive feedback in return for payment is absolutely inappropriate.