1. I regret buying. . . is a very frequently employed shopping-related expression used to refer to purchases a shopper is not happy with.
2. Why don't you take it back? is a commonly asked question that usually serves as a suggestive response to a shopper's complaint about a purchase. By asking this question, the speaker urges the hearer to return the item s/he is clearly not satisfied with.
3. I wish I'd never bought . . . is another commonly used expression of complaint.
4. A good buy is used to refer to a purchase that the shopper is very pleased with--usually a great/discount deal.
5. The best deal refers to the cheapest and most satisfying purchase price.
6. The Complaints Department, as the name suggests, is the section of the store that takes Customer complaints and grants requests for refunds, requests to exchange items, and so on.
7. A refund simply refers to the money paid for an item returned to a store. This money is given back to the shopper.