It is easer, mentally, to divide a separate fee as 'the cost of doing business' and not really calculate it as part of the cost you are paying for X item. Basically every functioning business knows this. There is a reason my LCS does not raise their price 10.25% on every item, and then put a sign on the door that sales tax is included in the listed price. Separating it as much as possible makes it easier for people to not have to think about, or if they do, to account it differently, even if it's not really rational to do so. I would imagine, though do not know how to prove, this effect is magnified when you've got a minute or two to decide whether to go another bid up. I also suspect, based on the surprisingly large number of jokes in the hobby about drunk bidding, that many are not exactly thinking clearly while inputting their bids on auction night
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Sometimes, I wish man lived to the Aristotelian. For good and for bad, we simply do not. While I don't have a whole lot of personal sympathy for people who don't read the T&C's they signed up for, it does not change what the auction house is doing and why this complication exists in the form that it does. It's a psychological tactic to make a larger number of buyers mentally divorce the fee from their bid, and also to their consignors to not seem like they are giving up X% of the bid, by adding this additional layer. There is a reason this is the norm, and it isn't random chance.