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

I have ended up with used cartridges when purchasing a used turntable. Got an AT 112SX cart with a parts only $50 turntable. I cleaned the contacts and bought a NOS Shibata stylus from LP Gear. Works great with period deck. 

I have bought a couple used MC cartridges from a reputable dealer on US Audiomart with no problems.  He seems to collect cartridges, breaks them in and then sells them, specifying approximately how many hrs. he's used each cart.  Getting a just-broken in cartridge for less than new is a good way to go, IMO.

I've sold 2 MC carts and purchased one used cart. Everyone including me is happy.

But generally this is a risky endeavor if you are not dealing with someone with a verifiable reputation. 

If you are contemplating something over $3K or so ($6K retail), you might consider asking J.R. at Wally Analogue to provide microscopy inspection/escrow services which would obviously need to be mutually agreed between buyer and seller. 

Bonus is you'd get a report that helps a lot with set up. 

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