What to call someone who buys only certain products (e.g. vegetables) from a certain store?
I've thought of the term "selective", but this seems to suggest that he/she picks his/her products carefully.
Instead, I'd like to place emphasis on the fact that he/she buys only a certain type of products from this store.
choosy
difficult to please because you are very exact about what you like:
Cambridge
I think "brand-conscious" or "brand-loyal" might be it. (Using brand to indicate the particular retailer, not necessarily the product which may not carry a noticeable brand.)