Any good online stores to buy old games from?

Play.com
Shopto.net
They don’t focus only on old games but Play has a pretty expansive PlayTrade section as well, which is a bit like eBay mixed with Amazon.

In my experience Amazon and Ebay are by far and away the best options. They’re almost always cheaper, and they have a wider variety/selection thanks to their notoriety. (I know this pulls away from your premise, but I felt it worth noting they really are the best option)

I’ve been buying Nintendo and Super Nintendo games off it for a year or so now, PS3 games should be no problem at all.

If you are looking for PS3 games,Gamestation in the UK is your best bet. Although, I would agree with @tedd. Please understand that gamers just like us are excellent sellers of classic or modern video games when dealing with eBay. I have purchased several classic NES games and new PS3 games from sellers on eBay, and all of them were in great condition. What games are you looking for on the PS3?

http://www.game.co.uk/

lukie games, not sure if they have ps3. I think they everything except ps3. Still might be a consideration for you.

glyde.com. It is a selling / buying site, and seems pretty nice. I’ve sold one game from there, never bought anything.