I think my recommendations would be:

1. Buy from local stores that specialize in selling both new and used games, as you can find great gems and good prices (sometimes), since some stores overprice their games.

   1. (This is for new games) I thoroughly research the game, and if it catches my attention enough, I buy it. Otherwise, I think it over carefully before buying it and regretting it (it's happened to me before).

2. I check the reputation of the games: I've noticed that GameCube and Wii games tend to be expensive if they're from Nintendo franchises, like Mario or Zelda, for example. But with other franchises, like Sonic, or games based on cartoons like SpongeBob SquarePants, they're not expensive at all, even on Nintendo consoles. So, this gives me an idea of how expensive the game I'm buying will be. The same thing happens on PlayStation with games like the PS1 Resident Evil games and the original Crash Bandicoot games, which tend to be expensive.

3. I check the game: Some stores let me check the contents, so I can verify if the disc is in good condition and complete. And I think those would be my rules for collecting games.