I know how you feel, I was worried about this at first, cause I do weekly artist showcases at the local Resort ....what I do is deactivate anything that I dont want to worry abotu getting sold twice, or if something sells at the art show, I immediately deactivate it, or remove whatever items sell on Etsy from my show, so as to avoid double selling something. Has worked pretty good for me so far.
I also keep some of my items JUST for the show and JUST for etsy, as I usually dont have time to list ALL my inventory...hope this helps!