There are a number of reputable ticket resellers out there: TicketMania, Undercover Tourist, Maple Leaf Tickets.
I personally have bought from both TicketMania and Undercover Tourist, and know people who have bought from Maple Leaf. No problems at all.
Shop all three for the specific ticket you're looking for, because if one place has the best price for one ticket, they may not necessarily have the best for another.
Also, tickets purchased from any source that's separate from a vacation package booking (a reseller, your local Disney store, or even a separate ticket purchase from the WDW website) can be added to your room key after you check into your Disney hotel. Just visit the Lobby Concierge desk and they can do it for you. (Though you can't do that for Annual Passes or any Florida Resident tickets)
-Rob