Totally mellow. A firm "no thank you" to the random beach vendor is enough. Bargaining? It's not always expected...but it can't hurt. San Pedro, for the most part, is pretty mellow and not much bargaining is expected. In Mexico, it sometimes seems that it can go on for hours.
Indeed. Which is why the sign I hold up on the beach in Playa del Carmen reads "Soy un agente del Secretaria de Administración y Tributaria! Venga!!"
...which, roughly translated means, "I work for the IRS! Come on down!!"