Thanks for this extra information. I suppose a corollary could be, “Always sell your common coins at auction.”
Except the seller's fees may negate any profit unless an inexplicable bidder war ensures. I think the message to never buy common coins at auction is generally wrong. Sure, there may be occasional bafflingly high hammers but if you're buying a common coin from a dealer it will probably cost more than if you'd gone head to head with the dealer at auction over the same coin. After all, the dealer in theory should drop out before you because he or she has to factor in the ability to turn a profit. If the dealer acquired the coin at auction in a well-priced group lot, that is a different story. You'd be paying the premium for not having to get stuck with the rest of the lot, and that may be worthwhile.
I agree. I've also seen plenty of common coins bring reasonable prices at auction and less than retail. It just depends on luck sometimes.