Many people don't bother to research the costs...also the thrill of the auction gets to them and they become too competitive.
I'm considering acquring a new camera lense. I figured I'ld check eBay to save some money. I had the brand new cost from B&H, and used that to decide my maximum. I try not to bid until the last 30 seconds or so to avoid driving up the price, but I was watching it. I was amazed again when I revisited just before the action ended. I figured my new price was about $320 including shipping. The winning bidder bid $311 plus $15 for shipping for a used lense with its actual condition pretty well unknown.
What is wrong with people? It appears they bid without attempting to determine what the item is worth. Color me perplexed.