Warehouse is all Amazon returns, AFAIK. But you can do used sales off of the main purchase page, and mail in your stuff to have it Fulfilled by Amazon. I've been looking into this quite a bit lately since I have about $7,000 of Canon gear to sell off.
I think I'll stick with Amazon. Ebay's 30% seems ridiculously steep.
For $7,000 of Canon gear, 30% is surely steep. I think you should just sell your stuff directly on ebay. They just charge 10%. I think ebay valet's customer base is people who have something around but don't know its value and just want to get rid of it.