I have not bought from or sold to TMR, but I suspect it is growing in popularity as a buyer because of the aging demographic trend of audiophile hobbyists. When I first got involved in this hobby twenty years ago, lugging large, unwieldy boxes that often weighed more than 80 pounds to FedEx or from my building’s mailroom was easily doable. I practically thought of it as a de rigueur aspect of the hobby that was necessary to find bargains and sell them at a minimal loss if they didn’t float my boat.
Twenty years later, my ability and willingness to do what I used to do, e.g. driving to the next state to pick up a 90lb VAC Phi 200 from a seller or carrying a similarly heavy Plinius amp to FedEx a few days after buying it used, are gone. Thankfully, I’m very happy with my current setup, so I have been fairly inactive in the hobby for the last decade. But I can understand the appeal, especially among older audiophiles, of selling to TMR, which takes most of the legwork out of selling used audio equipment.