Do yourself a favor and visit that local store instead. The people there will be able to help you not only get into collecting for the N64, but can help you with what you should get for it, not only as far as good games, but also accessories. The N64 has a controller that doesn't have the best analog stick in the world, so they can help you with that as well. Also when it comes to buying games, price charting is your friend.
Don't bother with dk oldies or sites like Amazon. They are there specifically to rip off customers who want to get retro games but don't know how much to pay. Your local store might not have the selection they have, but you can actually see what you are buying, and the website will often take advantage of that by selling at inflated prices and them sending damaged cartridges.
Finally, keep in mind that these older systems are made for CRT tvs. If you don't have one, look strongly into something that you can use that will connect your Nintendo 64 av cable to hdmi, otherwise it's going to look awful on a big screen tv