Though I do not have any first hand knowledge of the Cos Play sub culture, I was once interested in a girl who was heavily into Rennasiance Faires and Society for Creative Anachronism.
She and the others in the group were crafty and sewed their own costumes, wire wrapped jewelry, embossed leather items etc.
I vaguely remember that she and some others would wear partial costume around campus on occasion. For them, it was purely an attraction to the style and a chance to show their hand work to interested people (usually theatre majors and counter cultural types) rather than blurring reality with fantasy.
With her in mind, though it might not be common for those diving deep into cos-play to wear partial costumes on a day to day basis, it might not be strange to the extent that alarm bells would be triggered either.
That could be espescialy so at a garage sale featuring items that cos play participants might be interested in. Thus, a certain number of cosplayers were expected with the possibility a deep diving customer or two.