There are 4 types of carts to look out for when collecting a full set
1. Rare games that are in demand. Either because they are good or have some hype behind them. These will be the most expensive. Buy these now as they will just keep going up in value.
2. Expensive games that are not rare (ie Mario 64, Zelda, paper Mario) you should wait for a good deal on these as there are lots of copies around.
3. Rare games that are cheap because they are crap and the only people that would buy them are those looking for a full set. If you see these cheap buy them right away.
4. Cheap common games. Try to buy in bulk.
For type 1 hit up eBay or forums and pay market value for them. They will just go up in value later. If you happen to find a second copy in the wild resell it
For type 2 and 4 try and find these in lots where someone is selling their personal collection. Wait for a good deal. Keep what you don't have and sell the rest. You can make money and get some free games.
For type 3 hit up eBay or forums.
That's how I'd go about it.
