This has actually happened to me twice. I showed someone a comic, and they thought I was just giving it away for free and walked off with it. I felt too awkward to challenge them, since I honestly thought it was a misunderstanding and non-malicious both times, so yeah. Free comic, I guess.
After the second time, I made a post asking for advice on how to handle that situation on an Artist Alley group I belong to. After I described my sales method (illustrated above), I was advised to NEVER actually hand a comic to a customer like that, as the body language reads as giving it to them rather than selling it to them. So lesson learned! If you wanna look at the comics on my table, you'll have to pick them up yourself.
For the entire run, please visit www.CosPainComic.com!