When I worked at Wherehouse music (formerly Blockbuster Music, now closed), my manager became the manager at the Wichita Buybacks. Before they had an internal software system, they would go to ebay, amazon, and other places to manually look up prices. They would find the highest price and sell the CD, DVD, or game for 10% off the highest price. So if some knucklehead was trying to sell a limited edition of PS1's Harry Potter on Amazon for $189, Buybacks would sell the same game for 178 or so.
Related note: I remember I sold them a bunch of VHS tapes. I think there was 30 or so tapes, and they gave me over $150 for the lot. Score!!