I did a variation of this my freshman year, by actually buying about $10k of books from students on campus and then selling them on Amazon. We made about $3k all told... I guess the author is right that it didn't "really succeed" and become a huge business, but I definitely learned a ton. (And notably, Chegg is in this space and seems to be doing well enough to get Kleiner Perkins to join their $25m series B.)

THERE IS NO MONEY IN T-SHIRTS.

Nonsense. There's a lot of competition, but plenty of people are making tons of money.

Anything where you plan to make your money exclusively from ads

Bashing ad-supported business seems to be popular these days, but somehow those people are overlooking the fact that Google made $21B last year doing just this. Making significant money from ads requires getting to scale, but if you have the drive and the right mindset, getting there isn't as hard as you might think.