Crafter here. I sell painted glassware. Good advice on the college logos. Yes, they are copyright protected but nobody seems to care at the craft shows I do. Good luck with it. The good thing about your inventory is that it won't expire. You can carry over merchandise from season to season. The more the rock has a theme... school logo, profession, hobby, holiday, special interest.... the more likely it will appeal to someone and sell. Also, take special orders if you can with payment up front.
Sometimes you won't sell much because the craft show itself is a dud and not enough people attended. Don't get discouraged. Ask other crafters what the good shows are, they will tell you.
Keep painting, it is a learned skill. The more you do it, the better your work will be. Don't be afraid to experiment and fail once in awhile to get better. Keep us posted on things :-)