It is a misnomer that the reason why games are not as easily found as years ago is because of the collectors. It seems like there are a lot of collectors because that is what our predominant viewpoint is. But you have to understand the history of how we got to this point, and you'll find that it has nothing to do with collectors buying up everything at all. In fact, I am under the belief that there are less collectors now than there was 10 years ago. The reason being was that during those early years, it was before games were a known commodity, before the "retro" boom.
When that boom happened, resellers were buying up games for sale. Then the price spikes happened, even for common games. Then what happened? People got out because it was too expensive. Many people got more into emulation or digital and away from physical games.
Now we are in a situation where you can't find everything on ebay. All those games from years ago didn't go anywhere. They are all in warehouses and storage units of those resellers from years ago. The ones that did not plan for the de-valuation of the common titles. Used game stored went out of business once they found they couldn't make it on game reselling alone.
Why do I think that this is the case? Because there are still those old resellers still around that have all those games they hoarded 10 years ago. Piles and piles of common games still marked at 10 year old prices, and thus they sit there forever because there isn't anyone willing to buy them. Any of the companies that folded up, or shifted their inventory would likely have put the product that wasn't selling into storage. It may take some years for that product to filter back into the secondary market again.