Sometimes sellers are taking advantage of the situation, as well.
I'll give an example. I purchased a Macintosh LC on eBay, which pictured the infamous Apple IIe PDS card (which often sells for between $250-400 on its own, if it's working), for a buy-it-now of $125. The LC itself (being non-working) is worth no more than $50 at most, being non-functional. Who knows if the IIe card worked or not, so I figured I'd "take a chance" at $125.
When I received it, the IIe card was missing. I contacted the seller. They said they had a pile of these computers, and they took one photo of one of them, seeing as they were all "identical", and listed them with all the same photo. They were surprised they all sold so fast.
I looked back at the seller's history, and sure enough, I found several LC computers, with identical picture, which seemingly included the IIe card. I guess they listed one, it sold instantly, they listed another, and another, and another, and I had bought one of those "others". They had sold a pile of machines in the matter of hours.
So, sometimes a "deal" isn't any deal at all, and it's also very much "buyer beware". If I had been "the good neighbor" and simply let the seller know that the machine was worth more, and "here's another $100 for your trouble", I would have severely overpaid for a non-working, incomplete item.
And, to be fair, this has happened twice to me now (a Mac LC with IIe card pictured). It seems some sellers are either completely oblivious, or completely aware, of the value of some things. In terms of eBay and a buy-it-now situation, if the seller prices something and has it listed for public purchase, that's their business and not mine to tell them what they should be selling it for. After all, you might not be getting 100% what you think.
For all I know, this seller I mentioned had a pile of Mac LCs, all with the IIe cards inside, and someone told them about the IIe cards being worth that much. And then after the items all sold, the seller removed them prior to shipping, and resold them all later. I'll never know.