Here's the deal from my own experience. We are sort of a heavy alternative rock band. Our first show outside of my school was July 9th. We sold 20 some tickets to that, made $60, chalked it up to a semi-successful first gig because it was a venue very low down on the musical ladder and we had taken it to practice.
Our next show was July 15th. There was a touring band from Australia called Sunset Riot. We'd known about this show for 2 months or so and we were incredibly excited. To this, we sold 37 of the 50 we were given. This was higher than any band there, so we headlined that show (played after Sunset Riot -- seems they always have a local headliner with touring bands). The owner said he liked what he heard (which they always say) but at this point, I've figured out what he really liked was the ticket sales. He knew this was our 2nd show, and that we had selling potential, so he's putting us on the bill with a lot of touring acts. We sold 20 out of the 25 we were given last time. Then he booked us with Days of the New, where each band is expected to sell 50 tickets (band's request). IF we do that, we're put on the bill with Quiet Riot and expected to sell 75. These are all considered "band acts" by the venue. They are put together by the band, and use a bill of multiple locals to open up. All of the bands sell as many tickets as they can. Sometimes, if you don't sell any tickets, you'll be taken off a bill (seen this happen). However, you don't have to pay if you don't sell the tickets. They take the ticket money, subtract some of it to pay the local bands, and then split the rest of the money between the venue and the touring act. The venue was up front with us about this. If we book under a "venue act", however, they book a lot of bigger vocals and give them between 25 and 50 tickets to sell. Because the money goes straight to the venue, bands get half of the money they make selling tickets. There are no promoters to pay, so there's a higher payoff, you just have to do the promoting.
Anyways, I kind of got on a rant there. There were a couple of points I was trying to make though:
It doesn't necessarily have to be Pay to Play. Bands often request supporting acts sell tickets. Just assume this is to pay for the touring band, and that you will probably get a small cut (I'd figure $2 a ticket).
When an owner says they "love what I hear", they're most likely either being nice or what they really mean is that they like how much money you bring in. Unless he really loves metal, and you are one of the most mind-blowing metal acts he's ever heard, he's probably way too jaded to it to really love it. What this means is that he probably likes how many people you bring in and possibly how much money they spend while there. He may definitely appreciate good music, but he won't care as long as you're bringing in money for the bar. As evidenced by:
The selling 40 tickets deal, in my experience, is not a scam, especially if you don't have to pay for tickets you don't sell. However, it's still very important to sell all of them. This is a test. They're giving you 40 tickets to see if you can sell them. If so, they'll probably put you on the show with the national act because that's a moneymaking show. If they have 5 bands there that sell between 40 and 50 tickets, that's 250 tickets ($2500 gross) from each band, before the national act is even entered into the equation.
Be warned though, they'll probably be some washed out and hackneyed national act if it's a big band. We're playing with Days of the New and Quiet Riot. Are we excited? Yes. But the fact that they're playing small, hole in the wall venues with local bands nobody outside this city has ever heard of means that they're probably not at their prime career-wise.
It occurs to me also that "national act" can just mean somebody from somewhere else in the nation.
Anyways, hope that helped.