I’ve had good experiences with both TMR Audio and Spearit Sound. TMR gave me a fair price for trade on an integrated amp (maybe less than I’d be able to sell it for here on Audiogon, but remember they’re running a business and have to factor their overhead into the equation) and an equally fair price for the integrated amp I exchanged it for. I bought a refurbished CD player from Spearit sound as the result of a very informative and friendly phone conversation with one of their staff. According to their website, proprietor Ralph Spear is retiring. I hope this doesn't mean they're closing up shop.
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.