What to pay: When shopping whether in a shop, or from a beach or street vendor the first price you are asked to pay is not the real price. You are expected to offer a lesser price for the item, and then you and the seller will bargain by offering prices back and forth, until you eventually settle on a price that both you and the seller agree on.
Don't fall for the I'm Poor,look. Many of them enjoy a good lifestyle. They are better off (financially) then they appear. This does not mean that you should disregard them. They may have what you want. The prices may be a little higher but very convenient shopping on the beach. It does become tiresome after a while when you are trying to enjoy the peace and quiet.
What to buy: One thing that we usually do is make sure that we have a few pasos in our pockets at all times because you see a lot of small kids that walk around trying to sell you some chicklets for a few pasos. It's very hard to say no to the little ones so if you happen not to have any change, you feel bad having to say no.