I don't see the problem with it.
What is wrong with a band buying 100 tickets at $20, selling them, and then play the show?
Openers are 99% of the time not getting paid anyway. And if you're ON A TOUR as opener, you most likely will have to PAY to play (which isn't even the same as the situation you're talking about).
From a promoters pov I think it's a brilliant idea. If I wanted to make sure I could pay my headliner AND make sure there were people there to see them, I would definitely have local openers buy tickets to sell. If they then wanted to charge an extra fee on top of ticket value to whomever they sold the tickets too - fine by me, I wouldn't mind.
c.
If you're on the tour as an opener and you have to pay for a buyon the buyon generally goes towards things like your band's bus share etc, but it's usually stuff that matters to that band. Tours with no buyons require the band to bring their own transport, so yeah either way a band is throwing down money for touring. But, with a tour, you expose the band to more people than just one show. On top of that, if the buyon is 10,000 USD (or if there's no buyon and that's how much it costs to tour) for 25 dates, that's about $450 a night, which is about how much it generally costs to buy on to a tour. Considering that your band is also selling merch, and then you can actually make some money on the road. On tours without a buyon, the band is required to bring its own transport and there's usually a small $100 guarantee per night, so again the costs are justified.
The problem with promoters charging local bands a huge buyon for one show, is that you're paying way more money for this one show than you would pay for one show on a tour. If you pay 1500 bucks for the honor of opening for say Nevermore, how many of those people care about your band? How many labels have signed bands because they buy onto 5 local shows a year? If I was in a band, I'd rather we use that money to tour diy, playing dives, basements, anything, if we had to, and hire a publicist to spread the word. You could potentially reach just as much fans or even more, and be spending a lot less.
Anyways, that's just me. To each their own, and we'll just have to agree to disagree on it, as long as we can hug it out afterwords of course! hahaha