When I started remodeling the house, I went to the closest building supply store and talked to the manager. Told him that if we could reach an agreement on a discount, I would buy all the materials from him, if he had them or could get them. He refused, so I thanked him for his time and left.
If I need a small item, rather than drive the extra distance, I'll stop in a get it there. After a few months of me buying a few building supplies, the manager asked me why I didn't buy more from him. I reminded him of the discount I asked for and told him that another building supply store had worked with me. Just to rub salt in the wound, I also told him that I had spent over $40,000 at the other building supplier and that could have been his business, if he had wanted to work with me. He just shook his head and walked away.
FWIW, the discount the other place gave me was what they call the contractors discount. It's not a lot, but in the long run, it has saved me a few thousand dollars.
Always ask if they'll take less, ask for a rebate, special price if you buy a certain number of items, or a reach a certain dollar amount. The worse they can do is say 'No'.
