In business, in audio deals (even retail audio deals) -- and especially stuff like cartridges, remember little 'Dee Dee': DUE DILIGENCE. Check new and used prices, forum discussions of the item, ask the seller every question you can think of, and then ask a wise old audio friend if you forgot anything! None of this costs you or the seller anything but a few minutes of time. Picking it up in person? Check everything; make a list beforehand if you're easily distracted.
Another thing: even if the box it comes in doesn't look damaged, photograph the unpacking process (well, digital cameras aren't for serious photography are they?) Just do it, and don't be in such a hurry to plug your new joy toy into some juice. A little discipline can save a lot of ass! And post-mortems like this one would be unnecessary.
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