If you're dead-set on getting legit carts: Invest the few dollars in buying the screwdrivers and bits to open different kinds of cartridges. Not only does it help you verify carts, but it's also waaay easier to do a good cleaning on the contacts.
For expensive/desirable games: Only buy from sellers that open up carts for pics. When I sold my collection, every cartridge listing had multiple pics of the PCB. By doing this I actually sold many games in a few hours (even with high prices). Now not every seller is going to do this but that's honestly their loss. I wouldn't buy an expensive item without unique pics of the PCB to compare with a known good image.
VG_Addict wrote:I might mess something up if I open it up.
You can watch videos on how to open games if you think you'll screw it up. If you can operate a screwdriver you can open a game. It's not hard and you won't break it unless you're like planning on throwing it against the wall or something.