Sellers: keep stock honest
Other stores are putting their name beside your product. Keep stock levels accurate and ship on time, because a late order costs them a customer too.
Stores: credit the maker
Tell buyers who made what you're showing them. It builds trust, and makers notice who treats their work well.
Both: talk first
If you plan to change a rate or stop carrying something, say so first where you can. Small courtesies matter when a marketplace is built on shared shelves.
Sort out returns early
Be clear who handles a return before the first one happens. Buyers shouldn't get passed between a store and a seller. A short note on your product page saves everyone time.