When looking for any mic see if it looks like it has been dropped .(dented windscreens)
If it is a condensor mic see if there is rust or corrosion on it. If so stay away.. It means it has been subject to too much moisture and may be ruined internally.
With all mics,of course try it out. If it sounds good and has plenty of output, it should be OK.
Scratches are inevitable particularly on dynamic mics that have been used for live gigging. They are more hearty to withstand abuse, so give them a little more cosmetic leeway.
Hope that helps some.
Tom