When doing garage sales at my house, I always kept the valuable items that could be damaged by rain next to the garage. That way, if it started sprinkling, I'd be able to move the stuff inside the garage before anything was damaged. Did the same thing with my parking lot sales. Kept the nicer stuff closer to the tent or the box trucks. One time it really backfired on me when the rain was coming down sideways. For the most part though, it was an okay solution.
