My experiences:
I conducted an experiment a few years ago. I listed a variety of items across multiple sales platforms; Ebay, Agon, AA, US AudioMart, HeadFi. Items ranged in value from $50-$2000. Descriptions and pictures were identical across all listings.
I sold 95% of items through Ebay. I sold 2 items through US AudioMart. I sold 1 item through HeadFi. I sold -0- through Agon.
I had several low ball offers from Agon members, and lots of questions. But -0- sales.
I have had moderately more success buying through Agon. I was able to purchase an amplifier, preamp and few other odds and ends that are still in use by me. I also have encountered sellers who -refused- to sell to me because "I ask too many questions, and would be trouble to deal with later".
I recently listed a few more items on Ebay and US AudioMart. I just sold a very expensive vintage piece on Ebay- at full price. I sold a few pieces on US AudioMart individually, and as part of a package deal. In all cases, transactions were smooth.
Based upon my experience (and mine only), if you want the most exposure to the largest audience, and want to sell with the least amount of issues, then EBAY is the best venue. USAudio Mart may not have the traffic of other sites, but it is growing, and is now a very viable option.