If it looks to be in good external cosmetic condition, it probably has pretty low mileage. Excellent versions of the C330 are a little hard to come by as so many were used professionally.
All of the other tips above are good. Also, take a look and see if there is any "play" in the focusing screen. There is a bit of foam under the screen that may have deteriorated, which means your focusing won't be accurate unless the foam is replaced. Also keep in mind that the problems mentioned here are all fairly easy fixes. Guys on eBay sell the light seal kits, and any gummed up parts are usually correctable with just a standard CLA.
Also, if the C330 and lens have been in storage, do take along an LED flashlight and check for fungus in the lens. I bought a C330/80mm lens a few months ago that had a serious fungus issue that the seller never mentioned, likely because it couldn't be seen that well with the naked eye but was readily visible under a bright light. Thankfully the seller took it back, no questions.