Jawnson,
They certainly can be. Right now I have a shop with extremely high visibility, and that helps, but I'm using the same model that made me money at my old location, one that I don't think there's enough of in the marketplace. I like using chests to buy and sell, expecting only a small profit (mostly) per transaction. And, (with some exceptions) I don't stock the store -- I let others do it for me. If a chest doesn't refill itself, I adjust the Sell price (price I buy the item at from players) accordingly. I sell items to players at fair market, and I buy from them just slightly higher than the average. As long as I make something off a hands-free transaction, I'm happy.
In my old, low visibility location I did this with enchantments, because they supported a higher profit per transaction and that suited the location (ex. B100:S60). Now, I can do so using basic blocks/building materials (ex. B12:S8). That's fun, because not only do I not have to stock the store, I can use the place as my own self-stocking storage closet. I'm only refilling chests now and then when I'm feeling impatient, when I have extra from my personal stock, or when there seems to be a run on an item across the market.
Bear in mind, that I'm likely only able to do this using lower priced items because of my location, but I made back rent and then some without restocking
even when I was in a low visibility stall. I just had to do so using enchantments. In particular, efficiency and knockback did very well. I suppose they are just valuable enough for people to search them out in the market, but not so coveted that players with extra won't sell them to a shop-chest.
Anyhow, I'm mentioning all this just to note that there are models out there that might not involve you signing your life over to the shop. Also, everything Omnomivore said should be taken to heart.
levers