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."