you bring up a valid point Brian, I'm not sure there is etiquette as far as email goes anymore (or anything else).
folks get so many emails and many go to the "spam" folder without knowing gets deleted..
a few years ago I decided I wanted a Planet Ocean in the 42mm size and one popped up on a sales forum as I was looking. everything looked great including the selling member with 8000+ posts and I emailed him on the spot, asked one question and told him if that was a positive answer, I wanted it. I waited all day for an answer and when I got it he said he'd already sold it. I wrote back and asked how that could be, I responded within 1 minute of his post at 2am in the morning. he came back the next day and said, "I picked the 1st email on the subject and that guy wanted it, but I'm confused.. his email was 10 hours after yours"... we figured out that the first email he came to was the last email he got, he admitted I was first and he had made a mistake but he had already promised the other fella. I told him it was easy, he hadn't accepted payment or shipped it, tell the guy it should have been mine and sell it to me. he would not do it, no etiquette at all. BTW, after that happened 4 more times I vowed to never buy a Planet Ocean in this lifetime, must have been a sign or something.
politely write them again, or write them off as people that most likely you don't need to have any interaction with anyway.