Is this a bad idea? Can you really tell by visual inspection (of pictures, no less) whether a cassette of undetermined mileage is new enough trust? I’ve always been a little nervous buying a used cassette - especially nice ones for race wheels. Anyone have tips for determining whether something is in good enough shape? It seems even new cassettes have some shark fin teeth for better shifting.
This depends entirely on your financial and performance comfort. Cassettes are infrequent purchases, and as such are reasonably inexpensive in the realm of cycling hardware. Regarding performance, it all depends on how tight you maintain your drive train and how much slop you’re willing to accept. A used cassette will seldom be quoted at real mileage. Take whatever the seller is advertising and add at least 20% use.
Personally, I’d opt for a brand new middle of the road cassette over a used high end cassette for the same price . . . but that’s just me.
For less than $70, I’ve always bought CS-6700 or CS-6600 Ultegra cassettes. These things last forever. I have bought a used Campy cassette for the trainer, which worked just fine. They say that chains and cassettes wear together, so putting your used chain on someone elses used cassette might be a bit noisy. Also, putting a new chain on a used cassette could be noisy. Sometimes I see very lightly used cassettes - like 1 Ironman race on them, which will probably perform like new.
… Take whatever the seller is advertising and add at least 20% use…
I add a zero. 
but otherwise agree with your statements.
Hard to know as there are a lot of variables here:
We have Ultegra cassettes that seem to go on forever - with appropriate chain changes.
But a time spent beyond the chain’s life, in gritty, wet conditions, I am sure will wear a cassette down. Could you tell the difference just looking at it (cleaned). I am not sure that I could.
If it’s Dura Ace I classify those almost as disposable as a tooth brush. 105? I have over 7K miles on a 105 cassette/KMC chain and it shifts like butter…not so much as a hiccup yet. I’m sure somewhere over that 7K I lost a few seconds vs a DA drive train;)
I’m looking at a DA cassette with unknown mileage. I hear they wear out quick, but the price is CHEAP (way less than even an Ultegra).
Unless it’s less than $50 I would pass b/c you can snag new 105’s for that on eBay. Just picked up 2 for $105 shipped about 6 weeks ago. DA cassettes melt just by looking at them…unless you just have to have a brand name I would pass, personally. There is ZERO speed gain to be had from a DA cassette. FWIW I have been wrenching my own bikes since the late 80’s and take meticulous care of my gear. I would conservatively put the mileage comparison at 3:1 in favor of 105 over Dura Ace.
I’m looking at a DA cassette with unknown mileage.
Again - a lot of variables. You see it get mentioned here a lot that the DA cassettes suck-ass, and wear out in two rides! Well that’s not been my experience. But our cassettes get treated well - chains changed regularly, derailleurs tuned up, minimal crossed up riding, and drive-train cleaned and lubed frequently. If you bought one of our used DA cassettes and put it on your bike with a new chain, I am sure it would work very well. But if it was owned previously by a guy who never changed his chain, with out of tune derailleurs, with a lot of crossed up riding, rarely if ever cleaned or lubed the drive train and road in wet and gritty conditions a lot, that DA Cassette would be crap!
No. There are certain things not worth buying used and cassettes are one of them. End of story.
For me a used cassette would be fine for a winter beater bike, and maybe for use on the trainer on an old wheel set.
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For things like this, my rule of thumb is …can you trust the person who is selling it. If you know and trust them, go for it. If it’s some anonymous person on the 'net, take a pass. The old adage, if it sounds too good to be true… I always wonder when I see someone on a classified ad selling something that looks like an incredible deal, why couldn’t they sell it within their regular circle of friends if it’s so great.