Just because something isn't against the rules doesn't mean it isn't something to vent about.From what I read, the seller excepted a price deal. When the OP wanted to negotiate a date and time the seller said it was first come first serve.
The deal wasn’t finished. The seller did not go back on a deal. The OP appears to assume the seller would/could be at his beck and call and should meet anytime and date the OP wanted.
I don’t see how the seller is wrong here, although I am basing this from what the OP said.
A lot of things that are rude or lack of respect/courtesy aren't illegal.