When collecting video games, the condition of the game is extremely important. The value difference between a game in poor condition to the same one in new condition can amount to tens, hundreds, or even thousands of dollars in the case of very rare games. There are three conditions that are relevant to video game collecting: Loose, Complete, and New.
Loose: This refers to the game alone and does not include any other materials such as the original packaging, maps, instruction manuals, inserts, and more. A loose game has the least collectible value - collectors usually want to have everything that came with the game. A loose game will usually be bought by people who want to play the game, not collect. Many old games are not worth much loose, for example you can get most Atari 2600 game cartridges and Nintendo NES game cartridges for no more then a couple of bucks. While a game in loose condition is the easiest to find, some of the more rare games are hard to find even loose and can still cost hundreds of dollars despite missing many of the materials. But for the most, when you encounter a game in a flea market, garage sale, or thrift shop, game will usually be in this condition. If it is a rare game you should still buy it - and perhaps try and track down all the materials separately.
Complete: This game has been opened, but all the materials are intact. Game includes the original package, maps, instruction manuals, and any other inserts and materials that originally came with the game. It is important to verify that the game contains all the materials that were included with the game originally. If an insert or a map is missing, the game is no longer considered complete. Complete games can be harder to find then loose ones, because people usually buy the games to play them and throw away everything else except for the game itself. Complete games also cost a lot, and for the more rare games, if you find one in complete condition, expect to pay a fortune for it. Sometimes collectors don't realize that games can come with materials other then just the box and instruction manuals, so it is important to research all the items the game originally came with. You can do that by using search engines or going on different message boards.
New: Game has never been opened and looks like it just came out of the factory. In many cases this means game is factory sealed with the original plastic shrink wrap. This is the most desired condition for collectors, and the most valuable. However, a game in this condition is also the hardest to find. The rarest games can cost hundreds or even thousands of dollars more if they are in new condition - that is, if you are lucky enough to find them. If you do lay your hands on a game in such condition, make sure not to open the game as that will substantially decrease it's value. You can see by yourself by going to sites such as RarityGuide and looking at the price differences between the games in different conditions. This is the biggest mistake some collectors make and they later regret it deeply. And nobody can blame them for wanting to actually play the game, since many of the classic games are very cool. However, what I recommend, if you really want to play one of your games which you own in a factory sealed condition, is to go and buy the same game in loose condition for playing purposes. That way you still get to play the game, while keeping the condition of your collectible intact.