I see it from both sides as I know people who are in the business.
On one side a shop should NEVER advertise something for sale unless it's in stock and ready to ship. But most of these shops are still mom and pop stores.
So, on the flip side of the coin... they don't like to stock a lot of inventory. So you end up waiting for them to deliver your product or make it...
What they should do is state what the possible wait time is or could be. In the .50 world that's called the Mark Serbu "two weeks"! Which could really be two weeks but turns into two months. As long as you don't care to wait it's not a problem and if they happen to throw in a bonus, be thankful.
It's when they stop answering people get pissed off. Okay, I get that they're busy but to answer some emails at the end of the day isn't to hard. Keeps your customers happy and not cancelling their orders. I don't think that calling more than once a week or sending multiple emails helps though. I think it has just the opposite effect on the seller/business and all they want is to be rid of you.
If you get your order, thank them and either do business with them again or take it elsewhere. If it falls apart then warn others about your experience.