One thing is for sure: Koh Phangan is a lot cheaper than Samui when it comes to shopping. And again, Thongsala is cheaper than Haad Rin for the price of goods. However in both beachside towns you’ll find craft shops, jewelry and accessory outlets, hammock shops and, especially for those who come from more restricted cultures, fireworks!
Haad Rin shops and stalls sport lots of overpriced fluorescent Full Moon Party beachwear while smaller beaches such as Ao Thong Nai Pan Noi have typical seaside outlets selling beach balls, flippers and masks, suntan lotion, booze and of course those orange Rizla packets. Handicraft shops dot most of the main routes and are worth stopping at as their one-off products deserve attention while Haad Rin even has a sexy lingerie shop.
Generally speaking, despite most website overenthusiastic blurbs, Koh Phangan is not a shopping destination: Bangkok and Phuket handle that role a lot better, but you can find the basics here if you look around.