The capsule of a condenser microphone is the element that actually converts sound waves into electrical signals. If you want a microphone to operate reliably and sound as it was meant to sound, its capsule must be a factory original in good condition. Unfortunately, of the 15 - 20 studio condenser microphones I've bought on eBay over the years (Neumann, AKG, Schoeps and others), around half had defective capsules, but only two of their ads mentioned any such problem. Often the sellers honestly seemed to be unaware of it. Let the buyer beware.

Capsule damage can occur from mishandling (being dropped or whacked with drumsticks, etc.); that type of damage is likely to be visible in a good photo, though the seller can make the dents disappear by buying new outer parts for the microphone, often for far less than the cost of a new capsule. (Depending on the microphone type, it can cost from $150 to $700 to replace a capsule.) But permanent damage can also be done to a capsule over time when the microphone is used for close-up vocals without a pop screen. Gooey stuff gets deposited right onto the surface of the membranes, and eventually causes crackling noises or temporary cutting out of the signal, especially when it's humid or when someone's moist breath goes into the mike. You'll rarely see any sign of such damage from the photos in an auction ad; you probably won't notice it until you try to use the microphone yourself.

Capsules, especially in Neumann microphones, are very often replaced instead of trying to repair them. The skills, equipment and know-how to repair a capsule are so expensive, and the procedures are so risky, that it only makes sense to attempt it in special cases, e.g. "vintage" microphones for which the manufacturer has discontinued the replacement capsules (as with the Neumann KM 5x series). Substitute capsules from third parties vary greatly in quality. So-called "reskinned" or "refurbished" capsules are sometimes offered on eBay; it makes little sense to rely on their sellers' claims for their quality. Installing a new membrane usually changes the audible characteristics of a capsule even when the original manufacturer does it! A microphone with a "modified" or third-party capsule is properly regarded as a different type of microphone, with a different sound quality from whatever it originally had.