I think what GP meant is that when it involves money, suffering is nearly impossible to prevent. That's why you have puppy mills, for example. Most people don't know how the puppies are raised, they just see the cute puppy in the shop. The same way people see a pretty piece of meat in the supermarket and don't know its history.

Raising animals for meat is theoretically doable with no suffering (not sure about milk), but it's not happening in practice. With pets the situation is better - a lot of people adopt and some care about how their pet was raised if they buy it from a breeder.