Only buy tickets from the gate, or authorized sellers. There are a crap load of scammers out there and DO NOT buy someone's unused days on a park-hopper. They now take your photo at the gate when you go in and it is linked to the ticket. If the photo doesn't match, you don't get in.
FYI, you can buy military 3-day passes at the gate now and those don't count towards your 6 per year limit. My brother-in-law bought 12 so far this year.