Cartridge new or used ?


How do I know if a cartridge is new or used and number of hours played ? 1- If it is bought from an individual or 2- Could it be that a HiFi store sells used or a "Demo" without anyone knowing it? 3- Is a cartridge that remains on the shelf of the store for 3 or 4 years, retains its same performance or its internal parts, can harden and lose their flexibility of reading? 4- Is it easy with a magnifying glass to see the wear of a cartridge, if so what would be the best magnification to use?
audiosens
You can minimize the uncertainty of the "takes ya chances" part with a dose of healthy common sense and  research.

As I have said it is NOT for impulse buyers or the faint of heart...lol.

Works for me and keeps my pocket a little fuller each time I buy a used cart.

Good luck to all.
  I’ve had mostly good luck buying used cartridges. For my last example I paid 2K for a used Lyra Kleos SL.   To start with make sure you’re buying from somebody with lots of good feedback and make sure their backstory makes sense. For example if they claim 200 hours on a six-year-old cartridge, do they have multiple arms, why does it have low hours? 
 Then considering I’m getting it for 2K off of retail. So even if it has more wear I can retip it for $500 and be ahead of the game.  Then there are those that say it’s not the same cartridge after it’s been retipped. Technically they are correct but in my experience it’s still a very very very similar cartridge.  I retipped a Lyra Delos and I would be hard-pressed to have noticed that it was any different. 
 I refuse to pay the ridiculous amounts they want for new cartridges these days. 
@analogluvr .

I agree that the purists will throw a googly eye fit and wail long and loud about retipping one of the expensive MC carts claiming it is now not original at all.
But if YOU like the sound still then surely it is better than just junking what was an expensive purchase by someone.
I would have no hesitation in following the same path if needed.
After all 5 to 10k for a cart is way out of my league but 2 to 2.5k? For same item?
I will take a chance AFTER doing full research on the cart AND seller of course.
I won't do previously owned and used cartridges.  I do have friends who swear by it though and they have always had good luck.  Back in the day I bought and sold maybe 5 TT's and each either came or left with a cartridge.  When receiving a used TT and cartridge, I'd test the table with a worn LP.  If it sounded good to me, I would replace the stylus or consider a retip.