I think it'd be fun to break a social encounter into three phases, each earlier phase influencing successive phases.
1. Prep. This could be as simple as a Spot or Sense Motive as you walk into a merchant's shop, or as complex as an entire investigatory adventure.
2. Debate. Engaging the target in verbal combat could be a simple opposed roll for an argument (haggling over a price) or a static DC for simple objective (befriend the guard). Or it could be a series of rolls, like a static DC to "break the ice" and then an opposed DC to successfully engage in stimulating conversation.
3. Resolution. This could be simply the DM telling you the results of phase 2, or one more roll for when you actually seal the deal.
So for example, a sales call. In phase 1 you research the customer (Gather Information roll vs. static DC) and determine that he enjoys boating. So you read up on boats, with the intent to learn enough so that you can make conversation. In phase 2 you make a diplomacy roll against a static DC just to get him to spare you a few minutes of his time; you use boats as an icebreaker and get +2 to the roll. Success! Now that you're in a conversation, you outline the features of your product and the benefits the customer will enjoy, paying attention to what the customer says so you can adjust your pitch. Static Sense Motive check, with a +2 for the rapport successfully established in phase 1. Success! Phase three, and it's time to ask for the sale. The DC is determined by how much the customer actually wants this product. If you're selling ice to Eskimos the DC is very high; if you're selling water to a man crawling out of the desert you don't need to roll.

Your good listening skills give you a +2 on this diplomacy roll, and you get a further +2 for the success in phase 1 (total of +4 for both bonuses).
Success! Congrats, you've established the want to buy. Now repeat phases 1 to 3 as you talk about price.
Very simple or inconsequential encounters could just do phase 3. But debates with the King? Bargaining with Jabba? That'd justify the use of all three.