I'm a little perplexed/surprised by TMR and who sells to them.....

@OP - It's pretty simple. Some people have inflated ideas of what their used gear is worth. So they hold out for the price they think they should get and when they fail, they give up and sell to an online used goods retail business.

It's the fairytale of the Princess and the Pauper applied to Hi Fi.

As for TMR, they are effectively buying your gear at trade price so that they can apply a margin to it. They wouldn't have a business otherwise.

I’ve made several transactions with TMR and all were simple and pleasurable experiences. That is worth a lot to me compared to the hassle of selling equipment yourself.  TMR is a great place to buy high end equipment at great prices.  I purchased a two-year old amp that retailed for $20,000 for only $8000. It arrived in perfect condition and looked new out of the box.  Highly recommend TMR. 

@troutstreamnm 

Yes, that's good indeed for you as a buyer. Imagine what the schmuck that sold the 20K amp to TMR got for it. I'm guessing 4-5K.

My purchases from TMR in the past several years have crossed the six figure line. Most purchases were outright, and a few included trade fodder. In the trades, I understood that I could have gone on my own to sell that gear. But, this is a hobby for me, not a business and the ease of moving on from that gear by using TMR outweighed my perceived pickup from going it alone. In each case, TMR paid the freight, and took responsibility should damage have occurred during transit to them. Trades gain rewards points as do purchases, and I have been able to use many thousands of points towards subsequent purchases. I like there service and I am able to negotiate with my contacts thee to my satisfaction. In the end, I like their business model.

Yes, that’s good indeed for you as a buyer. Imagine what the schmuck that sold the 20K amp to TMR got for it. I’m guessing 4-5K.

The real point is, TMR sold the amp for $8K, and TMR commands a premium over individual sellers, so if TMR got $8K for it then the schmuck, as you kindly put it, might have gotten $6K or $7K selling it on his own. 

Therefore the "schmuck" was schmucked not by TMR but by his paying $20K for an amp no one wants.

Had he spent the same $20K on a McIntosh, the numbers would look very different.