It's really competative, but the hunt is fun.
I don't limit myself to anything particular...I'll grab anything that looks interesting. But, I also don't have anything that's really worth big money either. By the time I buy the game and refurbish it, I pretty much just break even.
It's pretty much impossible now to find anything that's going to net you big money--between the competition from other collectors, sellers thinking anything old is worth a fortune, and the fact that everyone is basically scraping the bottom of the barrel now in terms of basement finds, you're probably not going to strike gold.
Your best bet is to network with other collectors.