Anyone ever buy demo/used cartridges? Thoughts? Experiences?
I've bought plenty both new and used. Best is buying used from hobbyist audiophiles with stellar feedback who cleans their gear compusively (i.e. check the cart picks for a "dust beard", look for a clean stylus pic). Many of us also have large cartidge rotations, so no one cart gets too many hours. Never had any issues with cartridges bought selectively this way. Dealer demos can be OK-ish, but are usually not kept nearly as clean as the best hobbyists. And if you're buying from a gear flipper whose expertise caps out at fuzzy iPhone pics, good luck.
But if the seller is at all honest, and their feedback is to be trusted, they should be able to accurately describe the cartridge's condition, age, and performance. I've actually had a much higher rate of various issues with used (active) electronic components.