I have a few games that I know, personally. That will become more and more rare over time. I even have a list!
1. The Last Story and Xenoblade
This game, from the 4 hours I have played of it. Is going to get ultra rare if it is not going to be digital for a while. I honestly recommend someone to keep one in the packaging. I recommend that highly. I honestly felt bad for buying this game and opening it... it is that nice of a first launch thing they are doing. This game is coming out right at the end of the wii's life and now there is nothing other than that kirby collection next month. This game, along with launch stuff is going to fetch high prices later on... like... Earthbound levels of high. Also, LOW print run... so yeah... buy now!
This game, is not as rare as the other one mainly for the fact that Nintendo did publish this and did advertise it a bit. However be reminded that this is going to be rare as well one day, not Earthbound rare, but close. It is going to be safely guarded by video gamers due to high quality and expect it to go for a good amount later on. Remember, Wind Waker is going for 40 or higher!
2. Infinite Space.
This game is completely unheard of, but once it gets somehow famous enough due to high quality and great story it has. It is going to be pretty damn hard to buy. Low print run on this one as well. I recommend buying this one. Heck... I think it is 50 for the last copy on Amazon... No wait... 80 for the last two copies... Yeah... try to find them!
3. Tales of games in general
I recommend just buying them if they aren't going to be re-released. Tales of Symphonia can go for 40 bucks now... I so should have bought one for 15 when I had the chance... DAMMIT!!! But yes people. This just shows quality+low print run+no reprint=RARE! So yeah... buy them all if you can. I myself found a copy of Tales of Abyss for the ps2, practically new for 4 bucks... FUCK YEAH!!!
4. Any Japanese or game from another country really.
This is just in general. As long as a game is high quality but didn't sell well enough to make more. Just remember, keep it in good condition. Okay?
Other than that. I think that should do it for how to treat games and stuff. So yeah, obvious stuff! Just look at the trends and buy it! All it takes is for it to be so awesome it sells out! So yeah... that easy!