I've seen plenty of evidence of price manipulation on Ebay with both sealed games and used games. It's fairly easy to do with obscure games and doesn't require a massive investment on behalf of the fixer. You can see on sold listings the same person has bought up every copy of an obscure game available on ebay at the same time, and then started re-listing them for 3x the price they paid. After 3 months the evidence of them doing so has disappeared, and eventually some idiot comes along and buys one at the ridiculous new price (or they can even just sell it to themselves). Then everybody else starts listing their copies at that new high price as well.
That's literally how easy it is. People have seen what WATA are doing to the graded game market and started applying it to other market areas.
There's a reason Japanese games haven't seen the same kind of ridiculous price explosion American games have.