I see two things here, Jade. First, you found out why you really should not LOAN your scooter to anyone. Same for motorcycles, guns, airplanes, anything YOU value. Second, the dealer took your bike in trade so he will have to fix it before he can sell it. So why dint he fix it for you? He will not know how to fix your new one if it has the same problem.
You already know how to prevent problems from overfilling because several have chimed in on this forum. This forum will also be the solution if your new bike winds up with a goofy idle or a "Stig-spot" mid-speed misfire.
Karl