EDIT: Solved.

I just emptied the Pawn Shop, but I cannot unlock the achievement. Is something wrong, or am I missing something? Thank you.

You'll probably just need to wait a while. I think it took about 20-30 minutes for the achievement to register for me.

However, just as a backup, I'd suggest taking a screenshot of the empty shop and logging a ticket if you haven't gotten the achievement in 24 hours or something ^^

Thank you! I went back just now and it worked. ^-^