The abundance of a certain item is not the only factor in the RarityMeter rating of this website. If you read the
faq I think you'll see an explanation on how Rarity is deteremined. It includes a lot factors, among others popularity, demand, age and collectibility. I certainly wouldn't want the rarity to be determined solely on how often an item shows up on Ebay.
Let me illustrate with an example. Might be a little extreme, but it shows the point:
Let's say you pick up your pencil and draw a stick figure on a small piece of paper. Is that piece of paper rare? I mean, your stick figure is unique because there's only one such picture drawn by you in the whole world. According to a very strict definition of rare, your stick figure might be extremely rare since only one copy exists in the whole world
But will anybody want to buy your stick figure if you put it on Ebay? It doesn't show up much on Ebay because there is only one. There are more copies of PS1 Final Fantasy VII Black Label Misprint then there are of your stick figure. So which should have a higher rarity, your stick figure or the PS1 Final Fantasy VII Black Label Misprint ?
Well your stick figure has no collectibility value and nobody will want it (unless in the future you become real famous). But the PS1 Final Fantasy VII Black Label Misprint everyone wants, so even if there is more of them in the market in a given time, there is also a lot more demand for them- so there's actually less of them in the market then your stick figure since for the FFVII demand exceeds supply. (does this make sense). In fact, I can guarantee you that most likely all those FF7 games you see on ebay will get snatched, faster then some other obscure game which noone wants even if it appears less in the market.
I can say that I've been using the rarity lists and price lists here for a while I've found the RarityMeter very accurate and reliable so far.