Is it alright for me to plug some artwork I'm trying to sell over at Cafepress on this site?

On most forum sites I've used people use links in their signature to flog their wares, then look for any excuse to post in the forums I think that would be fine, and it would also be fine to put links in the descriptions of gallery images ("Get this drawing on a T-shirt at http://...")

If you posted a single thread here announcing your CafePress store, I don't think anybody would mind -- but I suspect it wouldn't do much good, either. I think the other avenues are more likely to be fruitful.