When you negotiate with sellers or suppliers, does it matter if you are going to be using their services or products for the long term or the short term? The answer probably has to do with your goals and motivations.
If your goal is to craft the best deal for yourself, regardless of your relationship, then the one-time or long-term question does not matter. Say you need to get your supplier to sell the widgets at $0.05 per piece, because that is all you can afford. You are motivated by price, so you are either going to deal with whoever can give you the best deal whether you are planning to buy from them again or not or you will negotiate aggressively to get the price you want.
On the seller’s side, it matters whether they are seeking a quick deal for some fast income or whether they want to work with you over a longer term.
Say that your goal is to find one supplier for your widgets that is dependable and has a great distribution system. Your budget has a bit of room. You are motivated to work with a company that offers good service, and so you are going to negotiate on more than price. You are going to negotiate on many other issues and you may want to do so with provider that can serve you over the long term.
A long term relationship will have to outlast the current negotiation, which may mean that you may have to make more concessions or be less aggressive in order to allow the relationship to move forward. When you have a long term business relationship with a supplier you have joint goals and you seek mutually beneficial outcomes.
In a one-time deal, you negotiate on a specific set of priorities and you may do so quite aggressively, not looking to how you are perceived or what this negotiation bodes for the future.