Also, when I make classic game purchases, I only jump on those that still have the manual and box. If I can be bothered enough to keep that stuff, so can other people. Kids excluded.
Kids were the bulk of the market in the cartridge era though. A friend of mine says his grandma would throw out game boxes if she saw them lying around because in her mind it was garbage.
If I was into eBay I would be much more picky about boxes and manuals but I limit my collection entirely to in-person finds. I have to find it in a used game store or at a flea market or the thrill just isn't there. In those situations finding a loose cartridge can still be a major find (ie: Ogre Battle, Earthbound). It's all about what you enjoy about collecting. If you want a perfect collection then the internet is a great tool for that. To me it's not just the game but the story of where I found it, what it cost, and what condition I had to settle for.
My greatest find? Star Fox Super Weekend cartridge for only 3 BUCKS!