If you're talking about the actual Mexican Riviera (IE the Pacific Coast and Baja), then I would stick with US dollars. There are a TON of US tourists, ex-pats, and Californians with second homes south of the border, so it's a bit different than the Caribbean side on the country. In my nearly 60 years living in So Cal, and literally dozens of trips into Mexico, as far down as Acapulco, by car, plane, and cruise ship, I've never once purchased a peso. Everyone takes dollars and gives change in US currency. They aren't selling to locals, they are selling to Americans.
They actually want dollars over pesos regardless, and will (contrary to an earlier post), actually give you a better deal in dollars (just have an idea of the actual exchange rate because, no, it's not 1:1). Dollars are more stable, and they spend better north of the border. Beyond that, I wouldn't buy anything from anyone that wasn't legitimately within the port of call, or in a recommended shopping area, as you're not going to "negotiate" anything and will pay what they wanted to get from you anyway no matter how hard you pat yourself on the back, and you really do risk being ripped off.