One way would be to post and ask other DISers for opinions and experiences.
Another way would be to go to BBBonline.com and check the URL against their data base.
You can also check their website for any logos that show their participation in self-monitoring programs like the BBB Online.
As for greatorlandodiscounts.com, they're also known as Ticketmomma.com and BillyBoy's. If you're comparing their prices to other sites, make sure that you factor in the 6.5% sales tax that they don't include in their listed prices. There is no charge for picking up the tickets at their location in Orlando but if you want them delivered to your hotel or home, the charge is $13.00. Several other ticketbrokers will ship tickets for free.