Are you doing it in a gallery? When my work was displayed in a gallery, the first night was for the shareholders. They oohed and ahhed and bought some things. The next afternoon was for the "members", again, same process. That night, it was open for the public. The sold signs were left up since the walls would have been a little bare in spots if not because there was a process of agreement. (The gallery had a clause in which they had to agree with the pieces being hung)
If you are being displayed at a private institution or a joint art show, then you might want to bring a few pieces to swap out the sold pieces.
Thanks, everyone. I'm a marketer by trade, so I felt leaving a SOLD sign might be a good idea, but not having experience in this field, I wasn't sure. So I appealed to the experts!
I will definitely have other pieces on hand to swap out just in case a buyer wants to take with. I'm certain there will be no problem with that--as long as the swapped piece is of equal quality, which it will be.