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

I own both Nagaoka MP 200 and 500. Both are great cartridges and I think are good matches for the Technics S shaped arms. I have not had any problems with either cart or stylus. Both are on Technics tables now. 

ok to do but need to be careful, know your seller

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.