I can't find anything about it online, but going from the word of my adviser while I was studying in China, there was a woman there who did something similar to this and is now very wealthy. IIRC, she paid people to wait around in lobbies and sell their spots when they got called, so people could bypass the rest of the line. What you are doing, I would say, is more ethical because instead of skipping the line a customer is just reserving a spot. I wish I could find some news article about this, but I have nothing. Either my adviser was lying to me or my web search skills are diminishing. Anyway, good luck, I think it's a good service.

Go after the big city hospitals where there tends to be a really long wait time, like Chicago, New York, and LA. Also, try to work in text message notifications for 15 minutes before the scheduled time.