I've had four transactions with TMR and every one has been positive.. OP is right that they deeply discount offers to outright purchase gear. Consigning gear to TMR typically produces a bigger return, but you have to wait, of course, until the unit sells.
In the case of an outright sale, you get immediate cash. Consigning items to TMR gets you the testing of your equipment to make sure its in order and advertising on one of the most watched sites by potential purposes. You also get-this is big from my perspective-finality. If there are problems after the sale-buyer claims the unit isn't as it should be, etc., its TMR's problem, not mine. That's worth something to me.
On the buyer's side, you get the comfort of knowing the unit has been thoroughly checked, a 30 day return window, and a 90 day warranty. That's worth something to me.
So, my experience with TMR has been really good and, for me, they bring added value that's worth some bucks to me. Of course, TMR's pricing and terms is known up front and for some the services may not be worth the cost, but TMR seems to be very good at what they do.

