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

I never thought I would,

then I started with one, got lucky, then found Yahoo Japan Auctions, took a chance on 3 used, all worked out. Happily, those 3 came with OEM Headshells with azimuth adjustability.

Here, I learned about re-tipping, and am lucky to live 35 mins from VAS,

https://vasnyinc.com/

so I purposely looked for, found and bought two used with broken cantilevers, planning on having VAS re-tip them with boron cantilevers and line contact tips.

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Why?

I wanted to try an otherwise unobtainable, never had one, Sapphire Tube cantilever, and it’s Van Den Hul version of a line contact

Next, I wanted to get my hands on AT160ML which has unobtainable Beryllium cantilever with MicroLine stylus; AND they track at only 1.25 gms. After the 1st one sounded soooo good, I acquired 2 more, I read that only 3,000 were made.

I also was encouraged by the fact than AT’s MM replaceable stylus design allows currently available advanced stylus to fit my vintage body. 

Oh yeah, I forgot, I found and bought a JVC Victor cartridge with a NOS stylus to have for my JVC Victor arm; A rare NOS Stylus, for my Shure V15Vxmr body with beryllium microwall tube.

Also, I should mention, aside from the sound character, which is hard to know except listening yourself, I always check and prefer two specifications that indicate capability for superior imaging: BOTH wide channel separation, and as importantly, perhaps more importantly, tight channel balance.

Oh, yeah, Oh yeah, all of this saved me a lot of money and got me what I consider a nice collection of terrific sounding cartridges. See some on the windowsill here

I have since put light blocking material on the rear and top surface of the cartridge box

No I do not buy used cartridges. The only person I would buy a used cartridge from is me. 

I like variety to spice things up.  It is the only way I could afford top cartridges: Koetsu Corralstone D (maybe best musical cart), VDH Grand Cru, Aida Mammoth Gold (gold coils) with zero to extremely low hours). 
When purchasing, I highly scrutinize the seller looking for near to 100% positive feedback from at least 10 sales. Established dealers I’d usually trust to perform.

But since it seems you’re asking to save money on normal wear, interest in cartridge rotation seems minimal. So I’d simply try to purchase new at the lowest price or one with extremely low hours, then run the cartridge until noticeably worn. If stylus replacement cost is an issue, then I’d take a hard look at SoundSmith cartridges.  

If you can afford to buy a 1200G- you can afford $850+tax for a new one.

Used carts can be a great buy, especially higher price point models-$2.5K+ or so.

Naturally, one must do their due diligence in making sure who you're buying from.

That means more than just an email exchange.

My used cartridge purchases were always what was advertised.

Get the new Hana, AT OC9 or whatever. There are many great sounding options well under $1K.