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

Verification that the Coils are good.

Some sellers include photos of multimeters, showing the coils are good, and the meter indicates the resistance. 

It's nice if the two coils resistance is close to each other.

IF expensive, you could ask the seller to measure both coils resistance.

If a cartridge meets the following criteria, it’s unlikely to have anything wrong with it anytime soon, imo:

• Cantilever isn’t askew (sometimes a little can be tolerated - Cadenza Bronzes tend to have a bit because their cantilever is tricky)
• Doesn’t ride too low or deflect too much (suspension should be OK)
• Good left/right channel balance; strong center image (coils and lead connections should be OK)
• Plays clean through inner grooves (stylus isn’t shot)
• No apparent corrosion (from bad environment and/or sloppy user care) and not too much magnetic material / crud buildup on yoke / cantilever / coils
• Not sibilant. All VdH Colibris have some, to some degree - it's their design and no amount of alignment is fixing it. Can't say I've heard it in any other model yet. 

If seller has included enough stylus / cantilever pic detail that’s good - and sometimes you can spot whether that’s the correct factory stylus mount for that model. Once in a while I see ads where the stylus is clearly a 3rd party job, and this has not been disclosed. A Euro / van den Hul stylus mount  (or VAS, SS) looks WAY different from a factory Koetsu. Very easy to pick out.

Straight Stylus

I download pics, and crop and rotate them, that helps me detect even slight deflections from straight, this one, see the width of the black on either side:

This might not have been a problem, however, if the cantilever is slightly off to one side, that indicates potential wear on one side over a long enough time to cause the deflection to be permanent. Or, it came that way, new from the factory, or it got banged into the side of the platter, and is ever so slightly bent.

This was a stylus only, if it was with a cartridge, even better with a headshell I liked, it is very slight, then I probably would have bought it, had VAS check it for me, ...., 

IOW, you just want to be careful, I was tempted, passed, found another shortly thereafter, dead straight.

patience is an asset, having a rebuilder you trust is an asset.

Was given an expensive Sumiko Palo Santos with cantilever missing. Sent off to SoundSmith to replace cantilever and it sounds amazing. No way I would buy brand new. Now I would buy used carts that need repair when the retail expensive is over $3k knowing retip/rebuilt is cost effective.   

I bought a used Soundsmith cart & had great luck with it.

You take a chance but can get an inexpensive rebuild.

I’ve got another used Soundsmith cart on the way that will be here in a couple days.