I heard these work really great for inspecting styli. You just plug it into your PC!
You can easily do it yourself with a low powered "toy" microscope or stereoscope. Look at the shape and wear of the stylus tip. Look at whether the cantilver is straight. Look at whether the stylus surround is compliant (i.e. not hard) and whether it is cracked. For an MC cartridge, any further inspection would involve looking at the wires which would probably involve taking it apart as in a retipping. The cost of having somebody do this is probably not worth it unless you are actually doing a retipping.
If the rubber is cracked, it is definitely hardened. However, before actual cracking happens, you can notice subtle changes. When rubber dries out, the surface tends to get a glossy sheen to it. It's sort of like the "skin" that forms on top of a bowl of pudding that's been sitting for a few hours. On the other hand, some rubbers are made to have a glossy, hard surface, a hockey puck for example, so that's not definitive. You need to have a point of comparison, for example, a new stylus/cartridge that you can visually compare the old one too. Of course, there's always your ears. Does the cartridge still sound okay or do you notice a deterioration in performance.
I don't think you need to make this too complicated for yourself. If the cartridge/stylus looks fine, i.e. the tip is not worn out of shape, if the cantilever is straight, if there are no cracks in the surround, then that's as far as you need to go. After that, just be honest in your for sale ad. Say how old the cartridge is, how much use it has and price accordingly. Wear and tear from depreciation is taken into account in pricing used goods. It's up to the buyer to satisfy himself that the price is appropriate for the condition of the item. He just needs to know that the item is accurately described when put up for sale.
Shure sold a microscope setup for assessing stylus wear. Their 'scope was 200X, with two side lights to illuminate the stylus' surfaces that contact the groove. The manual that came with it had photos of new, slightly worn and worn out styli for each different type of stylus (conical, elliptical, etc.). You compared the photos to the stylus being examined. Even with this equipment, it took some experience to properly assess stylus condition.
I just finished reading a good article on stylus wear in a 1980 "Sound Canada" magazine. The writer used to work in the pressing plant of a major record manufacturer. He said that the quality control people could replace styli on test turntables when they felt it was needed. He examined rejected styli using good equipment, and often could not determine any wear. So he did some blind testing with QC people, and they could tell a new stylus from a rejected stylus without fail. His conclusion was that the best equipment and an experienced tech wasn't as reliable as experienced ears when it comes to identifying stylus wear.
I didnt know about that shure system Id love to see the manual. I have a B&L stereo 7 with a dolan jenner dual hologen light source that gives magnification of 140X (with 20X eyepiece) I have heard people inspecting with 50X but my experience is that you need much more magnification to really do a good inspection. I have found flats at 140X that could not be seen at 50X and were audible. If those QC people could hear wear that was undetectable under 200X that is amazing. Thanks for sharing that story.