TMR are making a living in the very tough used audio gear market. If it was easy, everyone would do it. Their prices are not too far above the general used marketplace and they, unlike sellers here and on USAM, offer a no questions asked return policy. I purchased a pair of very heavy and somewhat expensive mono amps from them, that I may not have purchased elsewhere without the option to return them if I didn't like them. It doesn't get much better than that in the used marketplace.
Regarding what they pay for used stuff, that is entirely their business based on their internal market research and perhaps even search requests they receive from their clients. I have traded in gear to them that I might have sold on the open market but trading it in was a good way to get the deal done.
