Similar thing happened to me recently, except I'm pretty sure I know who made the switcheroo.
I was at a baby shower and the hostess was showing off her brand new kitchenaid stand mixer and also trying to sell her much older, much inferior mixer. She talked up the new mixer so much I asked where she got hers and went to buy that one the very next day. Thankfully I peeked inside the box before purchasing, because I thought it might have been opened, although it wasn't super obvious. Imagine my surprise when I saw the exact model and color of old mixer the gal had been trying to sell the night before in the box from the exact model of new mixer she had. I guess I can't say 100% it was her since I didn't actually see who brought it back, but what are the odds? I was so disappointed.
I could share some doozie stories about ridiculous returns, and it's been years since I worked retail, I bet it's even worse now.