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

If you have an item that is not generating the level of buyer interest you had hoped for, trading it in on something you want from TMR can help you off-load the gear you are having trouble selling while maybe getting a bit of a price break on whatever it is you are interested in buying from TMR.  I have done this before. 

This has turned into great advertising for TMR...which certainly doesn't seem to have been the intent of the OP

@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.