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

No reason to be perplexed/surprised.  It is their business model and they have been quite successful at it.  I have been treated respectfully every time I have bought/sold from them.  Each of my transactions have been seamless.

You already have a couple of posts explaining why people sell to them.  People buy from them to get the stuff they want with exceptional terms of service that include a return policy you wouldn't find from any other seller of used gear.  

"Every pre-owned product we offer carries a 30-day, satisfaction-guaranteed return policy unless stated otherwise in the product description.** Additionally, every pre-owned product purchased directly at www.tmraudio.com carries a 90-day warranty. During the 30-day return period, a return or exchange can be requested for any reason." 

@mulveling 

You just need to know your basis cost (including transaction costs) and approximate purchase date. I don't know how others do this hobby, but I'm in the red for every component I turnover so it's simple matter of recording the lines for your "loss, no net profit" on your taxes. Had to finally deal with a PayPal 1099K from last year, and will again this year. A little hassle, but not too hard, and impacted my return 0 dollars.

My situation exactly.  I basically pulled all my purchases for items that I sold (where the buyer paid using PayPal) by taking screen shots from USAM, some going back several years.  Lost money on pretty much every transaction, with an overall net loss.  My tax preparer had no issues with it.

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.