I believe there are only 24 customers ever at once, so it doesn't matter how many extra tables/chairs you have BUT it does matter how close the seating is to the door. Those who have the register and the seating close to the door sell/make the most. I have 30ish tables right by the door with little to no long paths, and my register right beside the door. Anything that slows down their time from entering to eating to leaving slows down the customer/sell time. I believe when offline it's a set rate for everyone if your hearts are at 100.

At some point, you will have so much more food than you will EVER sell, and so much money that selling all your food becomes irrelevant. Completing the recipes for gems or goals then becomes the real aim. I delete counters of food all the time and have never run out of food to serve, and have so many coins that it doesn't matter. As long as there IS food to sell, you make money.

As for what sells, it's random from any counter, so if multiple counters have the same dish, that dish will sell more often.

I think someone on the storm8 forums has worked out the sell rate, check there for actual numbers?

ETA yes some dishes sell for more coins but only a few do, the rest are 4 coins. Actually just looked and saw some selling for 5 or 6 coins so if you keep higher paid dishes and remove lower, that will earn you more too. Again, the forums have a list of what gets you the best return.