I've bought a bunch of equipment used, but never in person. For a lens, when you get it if there is a return window or if in person, I'd look for the feel of the zoom/focus rings, and inspect the front and rear elements for scratches for miniscule dings in the coating. To be honest, miniscule dings in the coating will not effect normal images and probably won't even show up in a test shot but they might hurt the re-re-sale value if your buyer scrutinizes it ad-nausea. After that just test the switches and it's ability to find and achieve sharp focus accurately and repeatably, you might need to mico-focus adjust it back or forward a notch depending. If you buy a body used, one thing to consider would be to test a long exposure in very dark lighting to see if the sensor has any severely lit pixels, this doesn't effect normal shooting but if you like long-exposure nighttime landscapes, some sensors don't preform flawlessly at 100% crop without fixing some lit pixel areas, my copy of the 7D doesn't, but for normal shooting it's perfectly fine and I rarely ever use images from that body under any circumstances at 100% view. My opinion is that if the seller has a good rating it's probably worth it for you to take a risk and even buy the item sight unseen. Lots of good deals online. Good luck.
