[Snowman]

I buy more commons for my sets on ebay but most of my better cards, and by far the majority of my total spend is with auction houses.

My take on why auction houses outperform eBay is because they make it much easier to process large transactions. I've had several $10k+ sales on eBay that fell through simply because the buyers couldn't process payments. And these weren't bums trying to shill bid, a couple of them were surgeons. I've also been affected as a buyer on a couple of high end cards that eBay didn't allow me to bid on (they decided for me that I was "spending too much too fast" and said they were trying to protect me against myself, despite the fact that I had paid for every card I ever bought and my account was in good standing with 100% feedback. Yes, that actually happened. I missed out on a really nice 51 Bowman Mantle because of it, and the seller likely missed out on at least a few thousand dollars. These sorts of challenges aren't an issue with the auction houses. They offer numerous ways to pay and a much longer time window that allows you to move money around from one account to another. Also, I think having one big auction that lasts for a month or so instead of just 7 days is a huge benefit as well, because the most important variable in how much an item sells for is the number of eyes that see it. And too many good listings pass through ebay without the right buyers ever even finding it because they were too busy to check for it in that 7 day window.