As far as BMX goes anything "Hutch" is like gold, older (1980's) Haro, Redline, Diamondback, some Mongoose, and similar can be collectable but look for GOOD condition, originality, and as complete as possible. Also remember to ask if they have the original parts (You never know what they might have hidden away)
As far as mountain bikes go look for high end components and good name brands like Trek, Diamondback, GT, Specialized, and other brands with a "Cult following". Don't try to make money on Next, Magna, or Huffy bikes, the tires, tubes, and cables cost more than a new one and you'll loose money every time.
Check police auctions for cheap bikes, check scrapyards in your area (If they sell stuff out of the yard) My local scrap guy is cool and I have saved many bikes from there on the cheap and made good money.
Hope that helps a bit.