Guidelines for buying used speakers (as in, are there any?)

I cannot speak to any of the speakers but the Vandersteen 2s. That speaker model line has been one of the best selling, most successful speakers for many years for good reason. A very good design and the sock enabled Richard to cut costs on cabinet wood and place value in the speaker by including quality drivers and components instead. His speakers are not gimmicky, just good design. I have the 2CE Sig II and I had listened to about a dozen different brands before I settled on them. Also, since so many of the 2 series were sold, that is part of the reason you can find them at a good price. They were sold new at a good price and there are many out there used which drives the price down a little

Buying second hand has its challenges.

Zu Essence ... saw a set, good price very intrigued ... seller claims he's had a bad experience and doesn't want me coming to his house so I can audition them. Hmmmm

Zu Omen Def's - he'll let me audition but I personally only want to make sure they're working. I've never heard a Zu before, don't want to put his speakers through the ringers like i might at an audio store.

Vandersteen, Totem's - seen some that intrigued me online but that's buying totally sight unseen. Ug.

2psyop -  I have the opportunity to purchase a 20 year set of 2s, seller says they sound pristine, said he took care of them "ike they were "another child" and rates as being in excellent condition - 9/10.

What kind of a lifespan do these speakers have? When would one typically expect degradation on the sound?

Turn this around.
How would you feel about selling used speakers?

Age or the amount of owners are irrelevant condition is whats important as were pointed out.

Depending how old speakers are original foam surrounds can be a concern.  As can replacement surrounds depending on replacements, were they done with quality accurate duplicates made by a company for a specific driver or one size fits all flea market type surrounds. And were the replacements installed by somone reputable or atleast someone that cared enough to do a good propet job. This should reflect in asking price or make or break the sale.

Also wire coatings can dry out and become brittle and crack over time and cause a short in extreme cases.

Capacitor age might also be a concern, they dry out in time as well, some sooner than others. Poly caps tend to last longer.

None of these should become a concern unless reaching over a decade plus. Or depending on where the speaker lived its life thus far. Sunlight and tempatures can play some part in it.