(former gamestop manager)
There are usually multiple opened ‘new’ games for the different sections. The seals are added so people can’t use the excuse ‘it was opened when I bought it’. Since inventory comes every day, usually the back counter is in a state of disarray as you are opening up cases to prep them for display, which is probably what happened in your case.
Once (sometimes 2 or 3 times) a week the manager before the store opens has to scan every disc in a section to make sure it is properly represented (if the title is in inventory and there is no disc one will be created, and removed if it was sold.) When someone misplaces the container and can’t find it they’ll sell it without finding the one on the floor, which is how that happens.
Employees are able to check out new games, but the rules behind it are so ridiculous that most of the time they don’t bother. Plus since most of the employees are ridiculous gamers they end up just buying all the stuff and trading it in.
As for the rejections of accepting trade-ins, that’s just not true. The employee might be lazy or misinformed, I used to pull those tricks when I was indifferent to the job. Gamestop bought a game for 2 dollars, sold it for 10, you trade it back for 2, and they sell it for 10 again. They make ALL their profits on the used games (which is why employees always try to push used games over new --it benefits both). The only time they’ll be rejected is if they are in shoddy condition.
The system overall works well. It’s a pain for the customer and employees to have all the discs behind the counter (and a LOT more work) but it keeps inventory shrinkage down to less than 1%, which helps keep the prices and deals lower. I forget the exact numbers but if it’s over like 2 or 3% they’ll toss everyone in the store. Unlike music stores where shrinkage is like 10%.