Hey John, I told you! Your art is amazing and people will love your unique style. I just knew it. I understand you were nervous. Totally normal, but now you have an idea of what to expect. Congratulations on the sale.
Making buying super easy is really important, so that sucks about your other interested buyer. I do hope they come back. I'm glad you have your Square swiper ready to go. When I did my open studios, that made sales go fast and plenty. It was like a magic wand.
While you sit there, maybe you can just write and doodle, take notes, play with your phone, but make it obvious that you're available to speak about your work if they want to. I always say that to people right away when they browse, and then leave them alone. "Let me know if you have any questions." Of course, I give them a super friendly greeting first. I'm not much of a social talker, but I can answer people's questions if they have them. Seems to work for me.
Also, remember your favorites aren't necessarily other people's favorites, as you saw. Maybe you think a painting is not your best, but someone else might feel it is magnificent (because it probably is)! You are your worst critic.