You hit the main issues right on the head.... age for decaying caps and speaker foam.... both of those issues can be addressed though if the price is right and you know what you are doing. High end speakers around 15 years old may start having issues but not necessarily. Ask about cosmetics, distortion, and If they are the original owners. I've bought several used sets of speakers and had to refoam one pair after a short period of time but the seller wouldn't have known it would happen. Some people want to know if the seller was a smoker or not.
What to ask when buying used speakers?
I have purchased used components in the past and I can do this with confidence their functionality is well defined and easily verified. I am starting to look for a new set of speakers and will consider buying used. Assuming that I am doing this over distance and will not be able to inspect or demo the speakers in person, what are some good questions to ask to determine the real state of the speakers? Any red flags? Can used speakers have incurred subtle damages or wear that will affect the sound? For the sake of discussion, let's assume that they are less than five years old so we can rule out decaying surrounds and old caps, etc.