Scenario 2:

It's Friday night and P2 has 41 unique items that he wants to sell. However, he only has 2 retainers, meaning he can only sell 40 items (20 per retainer) right now. The remaining 1 item, even though it holds a value if sold (weekday=300gil, weekend=1000gil), returns no value to P2 as it sits in his inventory.

Enter P1. P1 has nothing for sale by his retainers. He borrows the 1 item P2 has in his inventory idle and agrees to split the profit 50/50 with P2 upon returning the item. P1 sells it for the weekend price of 1000 gil.

On Tuesday, P1 buys the same item for 300 gil from the MB, yielding a net income of 670 gil (less MB fee) and he has the same item in hand. P1 returns the item to P2 with half of the profit (335 gil) and retains 335 gil for his efforts. In this scenario, P2 yields 335 gil from an item that he still has and P1 makes 335 gil from an item "they never owned". Win/win!