Actually, Sayulita is much more Peso based than PV and the more popular surrounding areas. You can get by with US dollars but you'll get ripped off and I often find it to be rude to the locals. They're not interested in US dollar bills and if and when they do accept it, they are just doing it as a convenience to you. And as others have mentioned, its a smaller pueblo so they run out of cash in the ATM's all the time. Whenever we are there, I bring a ton of Pesos and a credit card and use the card whenever they accept it. And when I do hit the ATM, I pull the max just in case. Helps that my bank reimburses me for all ATM fees and foreign transaction fees.