Wow, nice. Would assume you'll do well with it - all the underground comix stuff has a really strong niche following. Shelton may not be as mainstream a name like Crumb, but might mean the stuff is all the more rare and sought after.
Looks to be early 70s just by the slim cut of the "large" tee.
I've never seen one auctioned, so not sure. But some of Crumb's shirts fly.
Jimmy J
(Please note: Legit checks I do in this forum should not be considered 100% conclusive; I'm simply giving a gut reaction based on the limited information provided.)
I would list that at least 150 in my shop. People are looking for underground comix stuff. It might take a little while for the right person to find it, but the buyer is definitely there!