Under Thai law, travel agents that offer ticket, tours, tourism services, hotel reservations in Thailand must be registered with the Tourism Authority of Thailand (TAT) to improve quality of service and help protect customers from fraud. Please check to make sure you are booking through a TAT registered travel agent.
In Thong Sala and Haad Rin vendors sell pretty much anything you can think of, and probably some things you don't need at all. You can try to bargain, but realistically, the prices are set. You may get a deal every now and again, but it's the exception to the rule. Remember that you're in a tourist area and that prices usually are above the level of Bangkok. The further you go from the ferry piers, the better your chance of haggling gets.
Main purchases you will find on Ko Pha Ngan include hammocks (check out "Hammock Home Gallery" in Thong Sala) and some of the local artists' works. Most of the clothing is of the variety that you will find in Bangkok, but generally it is a bit more expensive, as it has been imported to the island for sale.
Art galleries are considered to be a rising business on Ko Pha Ngan. Most of the places will offer variety works and services including custom orders and art reproductions. These galleries have reputations for affordable prices and fine quality artistic skills.