Basically you will want to round up the RPGs as they are the ones that will always hold their price and or increase in value.|
Dark Cloud is a good one as its valued very high to people who collect but is unknown to everyone else. It looks very much an early PS2 release by its case. so it usually gets lumped into 99p bracket in cash traders etc.
Dragon Quest VIII is another that is creeping up and is slowly becoming harder to find.
Wild Arms 3 and (especially) 4 if you can find them.
And immediately buy anything with Suikoden on it if you see it sub 20 quid. Even IV which is becoming more pricey despite not being loved.
Also the Silent Hill Collection is still valued quite high and its definitely worth getting hold of cardboard cased Ico and SotC. The value of these has dropped due to the HD version, but this will not last. Due to the HD collection becoming less and less abundant and the cases being unique you can guarentee they will rise much higher as time goes by.
Generally just look for unique games by known devs. You never know which will catch on, it only takes a few articles from retro game sites/ media to create a buzz. For example Kings Field IV which is highly saught after because of its link to Demon Souls. Spy Fiction might be one to track down. Depending on what success Swery can find with the Deadly Premonition franchise. If it blows up over the sequel then expect people go looking for past games.