Sometimes on eBay, you might find a vintage toy or other item that is in less than "mint" condition.  Anything that has a "vintage" or antique status often falls prey to various nicks, scratches and breaks, but vintage / OOP (out-of-production) toys are extremely vulnerable to damage, often as a result of normal play (although occcasionally certain action figures will suffer from "display wear", although this is much rarer, and generally easier to fix), and the numerous small plastic pieces that make up most toys can sometimes break or fall off with a minimum of rough-play, falls from shelves, etc.

Usually, when one finds an "incomplete" action figure, vehicle, play-set, etc. on eBay, the seller will let prospective bidders/buyers know about the defect; either through the item picture, and/or in the item description.  It has also been my experience that these sorts of items are generally priced lower than ones in "mint" or "complete status."

Example:  you buy a mixed lot of vintage STAR WARS toys, including a SnowSpeeder vehicle (from The Empire Strikes Back).  However, you notice that the SnowSpeeder is missing the cockpit hatch.  After reviewing the item picture & description (confirming that this discrepancy was in some way noted by the seller), you could then search eBay for the missing parts.

Granted, it may take a few weeks to locate, pay for and recieve the missing parts (I find things like this on eBay every week), but this is still a great way to get those rare toys at a much cheaper price!