Plain and simple, there are PLENTY of legal places to buy tickets.
A popular one is the actual park's websites. Similar to what Hotel websites are doing, since they don't have to pay a fee to a third party, they offer the tickets at normally the lowest price available to the mass public.
Of course, you have plenty of Authorized Ticket Agents, including many online Travel Agents and traditional Brick and Mortar Travel Agents.
And of course, you can find some great deals using coupons and other special offers, a good website to check out the best current deals available is www.MouseSavers.com
But when someone wants to "rent" you a ticket for a day, and requires it to be brought back to the original location, that is against the park rules, and if caught, you can lose your ticket, and whatever you paid for them. These are "shady" business and IMHO, not worth dealing with.