Dealers That Sell Large Volumes of Used Gear Online


I have purchased from a couple such dealers, and my favorite so far is The Music Room in Colorado.  They have been entirely professional, and most importantly, their gear always arrives well packed and in better shape than even their description would lead one to expect.  Their prices are reasonable, they are not gouging. I'm not affiliated, just a happy 2x customer.
peter_s
I've had a couple of good experiences with Underwood Hifi as well. Wally was great to deal with.
I did a trade-in deal with TMR a few years ago. They gave me a very fair trade price for a Bryston B-100sst towards a Primaluna Dialogue Premium.  The tube cage of the PL came in with a cracked glass and they replaced the whole cage within a week. Very satisfied customer. 
Audio Solution in Indianapolis has been great to work with for both new and used gear .

Audio solutions Indy is a great place. Their prices are a little high, but the staff is highly knowledgeable and they have more inventory than the other 8 audio stores within 150 miles of me combined.  Like 20-30 times the inventory New/used I have seen anywhere.  I stop in once a month or so and it takes hours just to look at what they have. 
Spirit is a nice little shop in Northampton, Ma. Bought from them a long time ago. Recent purchases from Audio Classics in Vestal, NY were all completely first class in every respect. Would buy from them again.