Do you buy used carts/needles? Is it risky?

I bought a vintage highly recommended MC cart from a vintage cart retailer. It wouldn't play so I sent it to my trusted re-tipper. He diagnosed the ruby cantilever was broken internally which wasn't disclosed when I purchased it. I'd stashed it away for about a year before I tried to use it so it was too late to demand a refund. I had him replace with a boron+micro reach stylus. Sounds incredible and is my favorite cart ever but I got to that point through the back door. I strongly recommend doing your due diligence and determining the return policy. And don't be like me throwing it in the drawer for a year!

I have bought and sold a significant amount of used cartridges. 

When I purchase, I buy cartridges that are expensive enough to warrant having a new diamond installed. So not the entry level moving coils. I look closely at the seller and want certain things

Factory cantilever. If its sheared off there is the potential of damage

A pristine body. Anyone who mistreats a cartridge installing it is likely to handle ti roughly

Straight cantilever, not skewed. 

A suspension that looks to be riding at the correct height. 

I typically play a used cartridge on sacrificial vinyl, often a used copy of Windham Hill piano and acoustic guitar music. If it plays well I keep it for a bit, I guess 200 to 300 hours or until I hear mistracking. 

At that point I send it off to a retipper for a new diamond. These days I use only Expert Stylius or AllClear Audio. A diamond only retip is usually $350 plus shipping costs. 

I only have 4 cartridges now, and these will do me for the long haul. 

Transfiguration Audio Proteus

Kiseki Blackheart 1 gen

Benz Micro Glider H2

Sumiko Blackbird Lo

I just bought my first used cartridge of consequence this past Friday. It was from a fellow Canuck Audio Mart member. Even though I checked the seller’s history on CAM and exchanged several emails about associated equipment, shared experiences etc, I was still a little hesitant. The seller was also able to send me video of the cartridge in use as well. I drove a couple hours to get a version of a cartridge I had desired for a long time but never dreamed I could afford.  Apparently, this example had recently been repaired and rebuilt at the manufacturer in Japan. It had a channel imbalance of about 4 db. The cartridge was sold to me for a substantially reduced price.

I am lucky ,I guess, because it is glorious… hard to believe how much more it extracts from my own , well known vinyl. Would I do it again? Maybe…?
 

Any cartridge once played is "used". Each album played front to back is about a 1/3 mile of cartridge wear...after 2000+ hrs all cartridges likely need a rebuild or a new replacement. I just had my rebuild service done by Peter at Soundsmith. If you find one you like on the used market– a rebuild is a great option at a price-point about a 1/4 cost of new; and they also make great in-house cartridges too. 

https://sound-smith.com/services/phono-cartridge-repair-restoration-tip-repair

Yes but, only exclusively from Japan. All the sellers I deal with are extremely honest and helpful when asked for additional information when needed. They also fully clean and inspect each cartridge before they sell it. Some special or collectable cartridges in less than perfect condition can be purchased for a significant discount. I've purchased a couple in this condition  eg. two SPU's with bent cantilevers and had them restored for the same amount of the price savings. Moreover, there are a lot of used or vintage cartridges that were only ever available in Japan which from my perspective here in Canada, is value added.