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

HI,

Can anyone offer up some suggestions or guidelines for buying used speakers?

Initially I thought 5 years old max, buy from original (and only) owner, half original retail.

But then ...

Found some nice Zu Omen Defs ($1350) but they've had 2 owners.

Also found some Vandersteen 2c signatures ($900 + $100 shipping) but they're 10 years old and, again, 2 owners.

So you can see my guidelines may be preventing me from getting very high quality speakers at a good price.

The Defs I can listen to and physically examine; the Vandersteens would be purchased online.

Thoughts and comments would be appreciated.