So is all demand-based pricing rent-seeking? Like selling gold or a house or any other scarce good for more than you paid?

I think that if you poke at the term "rent-seeking" with the definition of "middle men who add no economic value yet inject themselves into transactions", you'll just end up arguing about what "adding economic value" means.

That said...if you can do that, you'll probably find that some additions of economic value are far more defensible than others, so it shouldn't all be flattened in the manner you're suggesting.

I mean, scalpers add economic value, right?! They allow (wealthier) people who didn't stand in line at the right time to have a chance to purchase a ticket!