Only you can make the decision about which ingredients makes the best quality product to your taste; if people agree, they will buy it. If not, they won’t.
In my experience, there is a difference in the amount of water in various brands of butter. I cannot tell you what the numbers are, but I can tell you that if butter is the top note in something I am making, I will always go with the best because you’ll taste it. In our bakeshop, I have Cabot for our every day, doesn’t matter what I’m making - cookie dough, ganache, caramel, lava cakes and butter cakes. But when it comes to buttercream, we use only Keller brand. If I could afford to use Plugra, I would, but my client base can’t afford it because it causes my food costs to go up. If you have a warehouse club near you, see what they have for the ingredients you are looking for - their house brand of butter may be better than the supermarket type - you won’t know til you try it and see if it makes a difference in taste.
It’s more evident in chocolate, too. In your brownies you might be using cocoa. I get a high fat cocoa from Felchlin that makes everything it touches magical; but sometimes it is not the right choice (look up threads on the Wowie cake. That recipe works best with Hershey’s.) Passover brownies are phenomenal with the Felchlin and just ok with the Hersheys. You will have to try out different types of cocoa to see what gives you the best product for the price you’re selling at. You’re going to take a hit giving out product samples at the Farmer’s Market - you need to make them wonderful enough to get people to come back and buy from you each week!
I have a better product with the Gold Medal Bleached flour for my shortbread type cookies but I don’t notice a difference in other things we make so I use the “no name” flour from a distributor for the brownies, etc because it’s cheaper. Sugar doesn’t matter; buy the no name variety. This may sound strange, but I can taste a difference in cheap vanilla extract (not the imitation stuff) and the better quality ones. Especially in cream cheese frosting and caramel! When you bake with it, you can’t always tell a difference, but when it isn’t baked, you can.
So my long-winded answer to your question is: it depends. On the type of ingredient, the product you’re making with it and the price your market will bear.