You may or may not bring supplier-direct experience depending on the time you have spent working for private property owners. If you do have experience with this type of work you can say, "When I worked in private landscaping I did spend some time negotiating contracts with suppliers for large amounts of fertilizer and brick. I am very comfortable in that type of situation and stay up to date on going and competitive rates for landscaping materials."
If you have not spent time negotiating a deal with a supplier you can answer, "I have not had the opportunity to lead a supplier negotiation, however; it's a challenge that I would be happy to take on. I am a strong negotiator in my personal life and am confident in my abilities to work out a strong price or contract."