In my exerience, that was not a common belief among most witnesses. There may have been a few kooks (within the larger pool of kooks!) who may have thought that about all garage sales, but 99% of the witnesses I knew loved to shop at Garage Sales! In fact, their Friday morning (and sometimes Saturday) field service consisted of return visits purposely mapped among the path of the best garage sales!
There was a common belief, it seemed, that some garage sales had items that might be considered demonic ... like records and such, demonic symbols or books. They believed that if they had very obvious "demonized" items, then there might be a chance that some of their other items might be demonized. My mother would make us leave that garage sale and not allow us to buy anything. I only remember it happening a handful of times, though. Definitely not an everyday occurence, and definitely did not keep us away from all garage sales.
Did anyone else have this experience?