For T-shirts, have them available in fat-tall-guy sizes.
So many places just don't have them available.
There are options for quality; if you get the cheapy lightweight shirts and can sell them for $12, and are upfront about them being the cheap lightweight ones, you'll sell a pile and people will be happy.
If you get the expensive, heavyweight T-shirts, you'll have to sell them for $30+, and you won't move nearly as many of them, however, guys will buy them and have them for years and be very happy with them.
Just make sure to get them in fat-tall-guy sizes.
And hats. They'll be popular.