The difference between buying and shopping is planning. If you know what you want, you go to the store and buy it. If you don't know what you want, you go shopping.
Once you've decided what you want to buy, you choose the best place to buy it from. This ensures (a) you get what you want, in terms of quality and (b) you get it at the value you want, in terms of cost and service.
When you go shopping, you let each vendor suggest to you which items you should buy. While you might get the level of quality, cost, and service you want, you can never be certain. It is easy for marketers to influence your expectations when you don't know what it is that you're actually looking for. They can change your perception on the level of quality and service you should seek and the sale price you should be comfortable with.
If you want to be an average marketer, influence my expectations. If you want to be a great marketer, influence my planning.