When to sell privately, when to use auction house?

[Aquarian Sports Cards]

I would think speed comes into it. Unless you get an advance from the auction company in question you are going to get your money quicker from the private sale (assuming the listing doesn't languish for months)

I think on "commodity" cards, things that aren't truly rare, even if they are valuable, there's very little risk of not hitting a decent baseline number at auction as the demand is high enough to protect you. That being said I wouldn't want to be the 8th Red Cobb in a given auction.

There is more danger with an unusual item, whether it's a high grade example of a Mantle Card or something truly rare. The supply may be low, but the demand is also low so it's the auction's job to make sure those eyes get to it. If they succeed, then it may well outperform what you ask in a flat price sale.