You can get some good deals here, and there are loats of ticket booths on International Drive etc that will sell you tickets.
1 thing to remember though, is that many of the parks now use a finger scan entry system.
In the old days, you'd just sign the back of the ticket (or leave it blank and sell it to one of the booths when you returned home). When I went in 2007, Disney and Universal both insisted that we scan a finger at the turnstile when we inserted our ticket. You then do this every time you enter a park.
if the tickets already been used by someone else.......
if you buy from a ticket agency before you go it will be fine, but be careful if buying from booths in Florida