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

I've had excellent experience selling to TMR.

Profit?

My father once said to me that a business which never makes a profit soon ceases to be a business.

What about avoiding the 1099 that now is issued by Paypal for every item you sell where payment was handled by them from the buyer? Do you still see a 1099?  It's not fun when you are selling gear via Paypal that you paid for in the first place but for which you have no proof of purchase price.

insulation…. 

@simao is correct, I made an offer on a piece of audio equipment they had for sale, and they excepted my offer. They don't always get full asking price.

Agreed. many of their products also have a make an offer option. And they will entertain offers and bargain with you. And, as @mapman pointed out, it’s a guaranteed and risk free transaction. 

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." 

TMR offers an easy and painfree transaction with no risk, not top dollar.  

To avoid the hassle of actually selling something which has become much more a PITA than it once was.