Buying and selling is an essential part of HD. Items in the RSS are fair game. You are free to buy as many items you can afford or have room for.
That being said, I personally don't distinguish between shops from farms I follow and shops found in i.e. the paper. However I do try to help farmers who need help, if I buy something from them.
I think, it is a bit sad to even have this discussion, because the truth is that even after playing this game for many years (seven or eight years), I still get thrilled, when I manage to buy a goodie. I also put goodies up for sale every day, so others can come and find them. You give some, you take some.
As mentioned in the start buying and selling really is an essential part of this game.