All of the exhibitors had signs at their tables indicating where the were from, and one of them lived just a little to the west of me. I mentioned my collection to her, and asked if she'd be willing to take look at it to see if any of the buttons were collectible. She offered me a business card and told me to email her.
It took me a couple of days to get around to it, but I finally sent the email yesterday. The message included a photo of the buttons that I'd posted on my blog. It only took a couple of hours to get a response, which told me that (as I suspected) my buttons had no collectible value. However, she commented on the "cute little owl buttons in the bowl of gold metal buttons", saying that I might want to keep them because they were made of metalized plastic, likely from the 1950s, and interesting.