I like the idea but it would be super generalized and significantly off in rarity. since this is a collectors site most users understand things like sports games having little value and rare games being sought after so there is an actually quite large discrepancy between games in collectors collections and their actual rarity.
for example I personally enjoy collecting shovelware ds games because in most cases I can buy them for 50-70 cents a game and they are incredibly common. but the largest majority of the shovelware ds games I own are owned by less than 5 people. where as a legitimately rare game Breach and Clear for the vita (only 1500 copies were ever made) has 57 owners of it.
You are right, but this idea can be tweaked in order to reflect the most accurate information, not just in sports titles but maybe some other crappy games that where on shelves but barely sold or family games that no one wants to collect.
For example I own a game named Ballerburg for the PS1, in fact 5 copies of it. And why you might ask, well I saw them unopened on a flea market and I bought them for 2€ a piece to reuse the cases for other games that i had later to find out that it is a rare game on another site like this one, but I don't believe that is the case since I see that game all the time on the shelves of 2nd hand stores, but since barely no one adds it to their collection it's flagged as rare.
But hey that's why we're having this thread to brainstorm on this idea a find the best way to do it.
Thank you for your feedback.