This. I was looking at this and came to the same conclusion. Either use PayPal X, or split the deal into two transactions, one where the end merchant gets paid

This. I was looking at this and came to the same conclusion. Either use PayPal X, or split the deal into two transactions, one where the end merchant gets paid and one where you get paid (either by the user or the end merchant). The only circumstances I'd use the route suggested in the OP would be if the merchants provided the goods/services upfront removing risk from one side of the equation. In which case you'd be behaving like a retailer, purchasing goods from your suppliers (the merchants) and selling them to your customers.