Cartridge new or used ?

You need an inverted microscope with at least a 40X or higher power objective in order to see the stylus shape and wear. The diamond should never be "sharp;" that would permanently damage a record. The profile needs to be smooth and polished, otherwise, if it has sharp facets due to damage, it will act as a lathe and will scour and scrape off the undulations in the vinyl.

Personally, I never buy used cartridges. Too much risk. It's not worth it.

This is MicroLine diamond under professional microscope and this is the same diamond under macro lens for iPhone. When the diamond (stylus tip) is bigger we can see the diamond much better, this is brand new, never used Dynavector MicroReach diamond, all we can see is a sharpness of the unused diamond. This is what i mean @stevecham  

Here is another example of the MicroReach diamond on the unused Dynavector 23RS cartridge. It is also looks sharp, but it's brand new. 

And finally Replican 100 diamond (the most complicated stylus profile ever) on very lightly used Ortofon SPU Royal G mkII. 

All pictures taken on my macro lens. Diamond in perfect condition or brand new diamond looks sharp. 

Damaged or worn diamond looks different, but to inspect them we need a powerfull microscope, this is an image from the internet, the diamond is worn accodring to the poster.  

Buying a used cartridge is like buying used underwear and not recommended, encouraged, or suggested.

 @chakster   None of the photos that you posted have any meaning whatsoever. The first stock photo by AT is a generic photo with just one face showing...and none of the rear facets and the actual cut angles, worthless. The other photos are equally as worthless, showing a stylus at that magnification is totally meaningless...well done, lol.
@clearthink.  +1

To answer the actual question, none of us is qualified to judge the difference in appearance between a new stylus and one that is lightly worn. No matter how powerful the microscope. That takes experience and skill. Also, most brick and mortar dealers are honest enough not to sell you a used cartridge in the guise of new. And finally, a never used cartridge that is 4-5 years since manufacture is going to be just fine. Many of us own NOS cartridges that are more than 30 years old, and we’re happy.