Don't worry about it OP, probably a good experience to have a shitty job, get your pay, go home and be a kid.
Upselling is annoying. One of my earlier jobs was in restaurants, Chili's, one of the big chains, and we always had to upsell our crappy presidente margaritas which came in the fancy shake plastic cups and were $3 more. I just did it because I didn't care. Some people were happy to try something a little different than just the house Rita, but 60-70% just politely said no and that was it. It was moderately fun to try to come up with your own slogans to hock the drinks, but end of the day, who cares. Worst thing that happens is your manager tells you your'e not selling enough Tweety Bird or SuperMan mugs are your pre-shift meeting.
Retail at Six Flags. It is what it is. The margins are probably pretty slim at your average amusement park, so if they can get an extra $2 out of a customer, and then get a small corporate bonus on top of that, it'll make a different for that park or that manager, etc. Do your time and then if you get the Target job you want, go for it. Realize, though, there will be annoying things about Target too. Otherwise stick it out at Six Flags and keep applying where ever, if a better job comes along, go for it. Nobody cares if you're a teenager and you job hop from one of these jobs to another.
Edit: And the GAF/Gamestop employee thing is one of the most annoying ongoing complaints I've seen on this site since I've joined. You walk up to the register, you tell them what game you want, you pay money, you leave. If they ask you to preorder or get a GS card or get a used copy or a limited edition or a Game Informer sub, just say no thanks. If they tell you that the game you want or your console of choice is lame, you say "whatever" and keep it moving. That can't force you to stay and talk to them. They shouldn't be able to drag you into a fanboy war. One person's opinion shouldn't enrage you. You're the consumer. You hold all the power. A transaction is only as long as you wish it to be. The stories people post here fuming about how they were "mistreated" by GS employees who dared to tell them about upcoming games always make me laugh.
I'm with you on the upselling at GameStop, but I have had a GameStop employee refuse to sell me the new version of a game -- Buzz! The Trivia Quiz show game for PS3 -- and insist on me buying the used one. I didn't want the used one, I wanted the new one (it comes with custom controllers and I'd rather just pay the extra $20 or w/e for the new one). I don't argue with clerks or even entertain them... but in this case he actually argued with me about it, and said "wow, I wish I had your money breh." And I wanted to leave but GameStop was the only store that sold it in the area... kind of a niche game. He, absolutely, would not sell me the new one even thought that's what I wanted
and they had it in stock. So. I was like fuck this, and relented, Okay, I'll buy the used one. I buy it, go home, open it, and they sold me the wrong game... The Previous year's game. They were practically the same box year over year, except the new one had newer trivia than the old one which was a year or two old at that point.
I was pretty pissed about that and don't shop at GameStop anymore. This was like 2010 maybe. He was also wearing just a wife beater shirt and his buddy was standing next to the counter with him.