Look, I will pay more on a BIN for certainty, if the card is a toughie. The item I scored last week is one of the rarest regionals ever made but is also obscure:
1968 Atlantic Oil Davis. This card was the winning SP for the 2nd prize of $100 in the sweepstakes. I cannot believe there were more than a handful made and I have to believe that all that surfaced in 1968 would have been redeemed for the prize. I'd never seen one offered before. If this card had been offered as an auction at the price I paid but with a BIN substantially higher I'd have BINed it anyway.
Besides obvious rarities like the Davis card, I also check VCP to see how many have transacted. I BINed a card recently at substantially higher than VCP because there were only a handful of sales in any condition in the last five years and it had been on my want list for a while.