A coin door is an important component in almost all coin-operated equipment, such as jukeboxes, arcade games, slot machines, and snack machines.
Coin doors come in a variety of shapes and styles. A lot of older equipment used a square door, while the "over/under" style seems to be popular with newer equipment. Basically a coin door is just a black door in a metal frame that houses several coin mechs, and can optionally include coin return chutes and bill validators.
New coin doors are usually pretty costly ($70 and up at the time of this writing), but there is little reason to ever buy a new coin door. Used coin doors are much cheaper, and even broken ones can usually be refurbished for next to nothing. Usually simply taking them apart, cleaning them, and putting them back together will do the trick. If that doesn't work, then you can simply replace the coin mechs themselves, without messing with the door. New coin mechs start at $15 from the manufacturer, but sell for next to nothing on eBay.
The only reason to ever buy a new coin door is if you want some of the features that a few of the new ones have, like bookkeeping, and better security. Actually, security is pretty much the only reason to upgrade your coin door. You see, a lot of older coin doors could be broken into very easily. You can get a Ms. Pac-Man coin door open with a flathead screwdriver in a few seconds. The newer doors are much harder to break into, but even they are not foolproof.
Ebay and the coin-op related newsgroups are the best places to buy and sell used coin doors. When buying them, always get a picture first, because you don't want to spend 2 hours repainting a rusty door just to save $5. When selling them you should always repaint them (if they need it), test them, and then figure out exactly which piece of equipment they came from. Your eBay listing for "coin door" might only bring in $10, while your listing for "Williams Defender coin door - refurbished", could easily sell for $50, even though it is exactly the same item.