Because chaweng beach is the most popular place in samui many shops charge 2-3 times more than it's regular price.
Unique Suggestions: Bargain at least for half-price or compare from prices from other shops
Fun Alternatives: You can try Lamai or Nathon market
Avoid vendors that have no number on a vest that they all wear walking up and down the beach. These guys are not licenced and will probably try and rip you off.
Or the other possibility is that they could be an undercover cop trying to get you to buy weed or something, and therefore possibly set you up for a bust.
Unique Suggestions: This trap is pretty hard to avoid unless you plan to stay away from any of the beaches on Samui. Most of the vendors are regulars, after you have been here for a few days, you will get to recognize them. Just try and avoid the strange ones that are acting a little odd from the rest, you will notice them. Most of them will be dressed in clothing that will seem a little weird for the beach.
Fun Alternatives: The risk of getting caught as I heard from a few people is not worth the trouble. Stick to buying gifts or food from the normal vendors. It is never a good thing to be ripped off, no matter what you are trying to purchase.
sometimes you think this it is a reasonable price, but, do remember this is in thailand!
the price seems no rules, everyone can give a much different price, so try more shops before you buy and make sure you gonna bargain
Unique Suggestions: at least 50% off the price they give, trust me
Fun Alternatives: if you wanna buy, first go to a mini mart, many things has a marked price, you can take that as a referrence