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.