D’oh!
I really should start checking rarity lists before bidding on items on eBay. Recently a couple of old consoles were listed on eBay as an auction with a “buy it now” price. I didn’t think the consoles would attract too many bids (they weren’t particularly rare) but the BIN price was a worry - it was (IMHO) too much for the pair, but not so high that someone wouldn’t snatch the items before the auction ended. Normally I don’t bid too early in auctions, instead preferring to watch and wait so I can snipe at the last minute, but in this case I decided to put in a minimal place-holder bid to remove the BIN option. Bad move! 
When I checked the auction in the dying hours, I saw that the bids had skyrocketed to more than twice the initial BIN price. At first I thought it was a case of newbies experiencing a sudden rush of blood to the head, but had a closer look at the photos attached to the auction. One of the included cartridges looked a bit different to normal, so I did what I should have done on day one - I Googled for a rarity guide and checked up on the cartridge. It turns out the cartridge in question is an extremely rare item which I could have snatched for a steal if only I had done my research up front.
Moral of the story: Do your research before bidding on auctions and look closely at any photos to determine just what is on offer.