I am by no means a NES collecting afficianado, let me say that right now. However, collecting NES is something I do just about every chance I get, since I have no other expenses, basically all of my money goes into buying games, most of which are NES. It's something I love to do and I thought I would share with the world my official "strategy" to buying NES.
First of all, check EVERYWHERE around where you live. Don't rely on Funcoland or Babbages, all of my good finds came from places like pawn shops, flea markets and thrift stores. Here is a quick synopsis of each of the main sources of NES games:
The #1 place to look IMO, your luck could vary from region to region though. If you find an NES seller that gets new games in a lot, consider yourself lucky and keep going back. Other sellers may have a limited supply of old NES games they are just trying to get rid of, the prices are usually $2-$4 in my experiences, but some people seem to think $10 is a fair price.
Another great source for NES stuff, however, pawn shops all across the nation seem to stop buying NES stuff, it's happened to 3 of the shops in my town since I started collecting. Check them out before this happens to you. Prices are usually 4 or 5 dollars, that seems to be the universal pawn shop price range.
Funcoland or other game shops
These are a double edged sword. While some games are $1-$2 or even lower, shops like Funcoland have a bad reputation for rude jackass employees and the customer service you'd expect in a concentration camp. This has been my experience, as I have NEVER had a good trip to Funcoland, and few good trips to other dedicated game shops that sell NES. Often I'll be brow-beaten when I come up to the counter with NES stuff, and urged to buy PSX or N64 games. Still, for some people, shops like Funcoland are the only good source.
Very unreliable, and hard to weed through to find NES stuff. I wouldn't bother, but you never know. Worth a look, I guess, if you happen to pass one on the way back from the flea market or pawn shop.
We'll get into that later...
There are some things I have to remind myself while I'm "out in the field". First, don't get taken in by a game because it's complete and shiny, don't assume that every unliscensed game is worth a fortune (most of them are worth jack-crap), and remember to study the rarity list before every trip. There's nothing worse than buying a game you think is mondo-rare to come home and find out it's a C- worth $3.
Remember to buy with a balance of wanting to cash in on a rare game and wanting to play a fun game. If you see a Tengen Tetris or Dragon Warrior III, buy it for sure! But, say, you don't have Contra yet and see it and right beside it is a semi-rare like TMNT Tournament Fighters, my advice would be to buy Contra. It's your call, ultimately, though.
Now, on to buying through ebay. Ebay is a lot different, mainly because there are thousands of other collectors vying for the same games you are, so you're not at all likely to come across a deal like Dragon Warrior III complete for $2. However, if there's a game you want, and it isn't in demand, you can be sure to find it for a good price. The only downside to that is shipping, and then there's the fact that you have to wait a week or two to play the game(s), and by then you may have lost initial interest.
One of my little secrets to ebay is misspelled listings. Check for misspelled listings every now and then, when you have nothing to do. You'd be surprised at what you can find, and chances are not many other bidders can find the same thing. I once witnessed a ROB go for $9 because of a misspelled listing (and I didn't have the $9 at the time, DAMN).
Finally, REMEMBER when the auction you bid on is ending. BE THERE, right as it ends, and keep watch over it until the final seconds. Another good strategy is to not bid on the item you want until there is, oh, say, 5 seconds left to go, then put in your high bid. This is an extremely effective method of winning auctions and pissing other bidders off.
Well that's it, a basic outline of my collecting strategy. You most likely didn't need any of that advice, but oh well.