I'd say your first problem is that you're going for the $90 eBooks...
Go for sell-rate over margin every time...
Fuck, I wouldn't pay $90 for a REAL book unless it was part of my uni course, or an art book by an artist I REALLY liked (Barr & Ryden... awesome...)
Any important information in life can be found for $25 or less, because the cheap books are the exact same shit as the expensive books, they just don't spend shitloads of money on marketing it.
Anyway, you're in highschool, right?
Word of mouth networks are GREAT for the sort of shit that appeals to teenagers.
How many of your friends at school are incompetent fools that suck at using a computer any further than booting up Word or WoW? Be honest here...
There are affiliate programs for selling help-desk services for P2P use... Hell, I can hook you up with one...
Anyway, you tell your friends about the site you're running with something like that... The ones that aren't tech savvy buy in, love the service, tell their friends on MySpace and give it a personal recommendation because they know the guy that runs it (you). Bam! Suddenly, all of their friends that suck buy in too, and so on...
Yeah, it's got finite growth, but if you make 100 sales out of it through your school and the other locals, that's still better than the pocket money you get from your folks, right?
And the thing is, if the video game thing works out, you can do it with others...
Buy another domain, and set up one for music, one for mobile phone content, one for movie downloads, etc...