Lots of good advice here so far! If buying from a reputable dealer I wouldn't worry honestly, assuming they have a good return policy. If buying from a dealer, I would still like to know if the speakers were modified in any way. This can make them sound different than what you are expecting, but it can also lead to problems in the longer run. I've seen speakers with changes to the crossover or drivers which would put a lot of stress on the other drivers, but also once bought a pair where the previous owner drained all the ferrofluid from the tweeter to 'make them sound more detailed'.
I buy all my speakers used and I do always expect to have to do some small maintenance, such as glueing a surround or replacing ferrofluid. If you don't mind doing something like that with some pairs, there are real gems to be found for very little money (:
@mtbiker29 cats destroy grills, but I've also seen surrounds and cones with claw marks.