I've now made 2 different shirts and have been successful at selling them. Here are some things I have found in the process.
Roll your shirts into a tube, tape it shut with a piece of colored electrical tape. Use the colors to indicate sizes. When you are in the lot, the commotion (or possibly your mental state) makes digging out a size for someone a bit tedious. You'll be able to complete the transaction much quicker with these color codings.
White's are M's, Red / Orange is XXL
American Apparel are popular, fit well and are nicely made in the US. Fruit of the loom and Gildans are good too. It helps to have a nice quality shirt and is worth the extra buck or two.
Most people by medium's and large's. On a sausage heavy tour like Phish, you'll find more people buying XL's than S's, but I usually get the same amount of each since girls will buy things off eBay. And get a few XXL's. They'll be appreciated :).
What you don't sell on tour, will most likely sell on eBay. Buy a box of these and a roll of shipping tape. You can print your shipping labels via paypal. It's pretty painless to do the mail order part.
This will hopefully go along way in keeping you from being arrested / ticketed. Cops seem to think they can do what they want, and could probably force you to go to your car to get it, but being able to truthfully say "I don't have my id" will get you off in many cases.
If the cops do decide to screw with you, they will likely take your shit. I usually walk the lot / shakedown and will carry a small bag of stuff. If I need more / another size, I head to my car to re-up. This way I have less to lose if the cops decide to step in.