Yesterday, I took part for the first time in a vendor fair. Several of my author-friends have done fairs before, and the feedback I always get is that they’re hit or miss, either you sell lots of books or you don’t — and I should qualify that when I say “lots” I mean maybe three an hour. Since my local high school was hosting the fair and the money raised would be going to a scholarship fund, I thought I’d give it a try. And lo and behold, I sold lots of books!
There are things you can do to make your vendor fair appearances more successful. Here’s what I did:
1. Publicize the event. On social media in advance (if you like) and on the day of.
2. Create signage. I have a bunch of really cool plaques that I’ve made in the past regarding various contests that Baby Grand and I have placed in, but I needed some signs specific to this event. Keep in mind that your signs don’t have to be state-of-the-art. Mine were rather rudimentary. I created them last minute on Microsoft Word — I couldn’t access InDesign for some reason — and made them in, seriously, five minutes, but they worked: 
3. Bring with you the following items: table (be sure to know the size of your selling space and whether or not you have access to electricity), chairs (at least two — even if you’re alone it’s nice to have an additional seat, either for an unexpected friend or for your arm or purse to rest on), clean solid-colored tablecloth, sign-up sheet for your mailing list (even if folks don’t buy books, they often sign up, either out of interest or pity), a bunch of books (I brought 30), several Sharpies, a camera, and perhaps something to keep you occupied when things are slow (I brought my Kindle, although it never left my purse).
4. Be attentive, not annoying. Of course, there were people who refused to make eye contact, and I left them along. but for those I saw glancing at my table, I asked, “Do you like to read?” or “Are you a reader?” to get the conversation going. And it often did.
5. Keep your eyes open for additional marketing opportunities. As luck would have it, a caricaturist was located in the booth next to mine, so after spending the day watching him make lots of families happy with his drawings, I decided to take the plunge and now have a fun marketing image that I can take with me to future fairs or use on social media: 
Making 'Baby Grand' (& Other Literary Rugrats) 
I agree with you about the hit or miss. Not long ago, I participated in a book event where the location was inside a mall. This sounded good as you can assume people are shopping but the location inside was actually somewhat isolated. The idea for the event was good but not as well-planned as it could have been. I would definitely participate in another one but I would want to know ahead of time what kind/how much publicity the organizers were doing and how the fair was arranged. Of course, every writer would also do their own publicity but more is better! You are right on with all your points. So glad it was successful for you and love the caricature!