Definitely post the sign. If I don't see it, I'll assume it's up to me to suss out if you have pets or smoke or not. If someone across from you with similar wares and prices has a sign, I'll visit her first.
I have six furry kids so no offense is meant to those who sell from pet-friendly homes; I just know I'm a pro at getting bodily fluids out of textiles and if you are, too, I might be buying something your cat threw up on. While I might not notice, one of my own crew might, and then I'll be in trouble.
