I know the OP talked about selling games, but several others chimed in about buying.
When buying rare games, it's a pretty safe investment. So you buy Radiant Silvergun for $150 on ebay, receive it, play it... and discover that hey, maybe it's not your thing after all. Big deal, just ebay it back, recover your $150, and move on with life. Most rare games are like this, assuming they're in some sort of demand.
The only risk is in the case of a re-release which is extremely unlikely, ESPECIALLY on a dead system like the Saturn. But in the case of Panzer Dragoon Saga, come on... $100 is only like $35 over the MSRP of the game when it was first released. It's hardly that big of a markup.