The best way to know what to buy and to sell is to do some research. I recommend looking at the finished auctions. Here is the search that I use to find the highest selling used Nintendo 64 games.
You will see that these are some games you should pick up right away if you see them:
* Banjo Tooie
* Conker's Bad Fur Day
* Harvest Moon
* Killer Instinct Gold
* Mario Kart
* Mario Party
* Mario Party II
* Mario Party III
* Mario Tennis
* Mystical Ninja
* Paper Mario
* Super Mario 64
* Super Smash Brothers
These games all resell for at least $15.00 a piece. Some up to $30.00, and they can commonly be purchased at garage sales for as little as $2-$5. This is a good way to pad your income.
If you want to go more in depth with the Nintendo 64 venture, I recommend you compile a list of all of the Nintendo 64 games selling on eBay, and try to guess what their average selling price and sell through rate is.
After you have a list of what sells and how much, you can stop depending on chance to find the random Mario game at a garage sale, and start looking to other sources. If you know what sells, and for how much, you can make informed buying decisions. Look at craigslist.com to see if anyone is selling a Nintendo 64 bundle in your area. The 64 and controllers themselves sell for about $30.00 Add a couple of good games, and it could be a profitable venture.
Also, if you are feeling really on top of your game, you can buy bundles of Systems and games on eBay. Every day someone is cleaning out their kids closet and selling bundles of games and systems on eBay, when they could make a much larger profit selling the items individually. This is where you come in. Buy the complete package, and split it up, and resell each item to make some good money. There are quite a few sellers who do this, sharkkisser, ryleys, datsunfamily, to name three.
To see how these sellers get their products to resell, look at the feedback they receive as buyers. They all buy Used Nintendo 64 lots and resell them at a profit. Some of them have become Power Sellers just buying packages and splitting them up to resell on eBay.
You can do this! It is not that difficult, it just takes some homework, and a little bit of start up capital.
For more tips on what to sell on eBay, please check out my blog the eBay Selling Guide