blizzard is very fussy with there piracy protection.
ive heard of rumours that if a cd key is used on more than one computer at a time....and both of these computers attempt to log into bnet more than a set number of times then the key is banned. to me that sounds a little far fetched.
If the key is not legit, then 99% of the time it wont work on multiplay(bnet). if u did buy the game send blizzard an email. they will issue you a new key or unban the current.