[Snowman]

In my experience, it depends on the card and your reach. I've done some 5-figure private sales, but not a ton of them. Most of the buyers who frequent places where you could sell outisde of a major auction house, places like Facebook, B/S/T here, Instagram, etc., are trafficked by buyers who are looking to score a deal. They're mostly bargain shoppers who are looking to save on eBay fees. As such, they are generally content with lower-end copies of high-end cards. So if you have a 311 Mantle with some creases and a pinhole on it, it's a great way to transact it. The market is pretty well settled on what that card might bring at auction, and nobody gets surprised when the hammer drops. Same with SGC 1.5 Goudey Ruths or a low-end green Cobb.

The challenge with transacting directly, in my experience, is more with cards that have tremendous eye-appeal for the grade. Those cards have a much wider distribution of what buyers are willing to pay for them. Those are the cards that can have shocking (to some) hammer prices at auction. Put up an off-centered 311 Mantle in a PSA 3 holder with obvious surface wear and a small crease, and everyone knows what it will fetch. But put one up in the same grade that's dead-centered with no surface flaws and just some slightly soft corners and it will easily fetch 2x what the other one sells for, and sometimes even 3x. But if you try to sell this privately, you'll just have a bunch of bargain hunters wasting your time trying to buy it for whatever "comps" sell for (they don't know how to comp). It's a lot of work. I only collect cards with high eye-appeal, so I encounter this often. Everyone wants my copies of cards, but most aren't willing to pay what it actually takes to get them. They want that 2x-comp eye-appeal card for 1x price, or maybe 1.1x.