I learned my lesson the hard way, buying cymbals off of Ebay.
I bought a 17" Paiste Signature series crash. Paid $65 for it. Had a 1 inch, hairline crack towards the center. Couldn't even see it unless you put your eyeballs right up to it. Figured since it was so small, and so thin, it wouldn't go anywhere, and the dude said that he had been playing on it for a long time and the crack never spread.
I played that thing for maybe a year, and the crack spread tremendously, half way around the cymbal, very large, and definetly not hairline!
That cymbal is now a clock on my wall.
The moral of the story? Be careful what you buy on Ebay and really check it out.
And don't buy cracked cymbals on Ebay either, no matter how tiny the crack is.