A couple of thoughts without having read every post in this thread....
If I want to get a ballpark value for a card, I look at multiple sources....the price history here, Sportlots, ebay, the hated Beckett. I suggest not just relying on only one source when trying to place a value on a card.
Sportlots....I've recently set up a store there. If I put a mid-range or high price on say a Tom Brady card and there are 500 of that card on Sportlots AND several sellers have that card at the base of .18 or even say .25, who are the buyers more likely to purchase from? The store with a $2-$5 price or one with a 'bargain' price?
When setting prices on there I have to take in to consideration the prices from multiple sources, how many other copies of that card are on Sportlots, what the other asking prices are, and what my main goal is with a store. Do I want to get as much money as possible? If so, maybe ebay is better as people seem to over-pay there more often. If I just want to get some of the 15K duplicate cards out of my basement (which is mostly me) then I set prices toward the lower end to get the stuff out of my house.