The thing to buy in Mae Salong is the excellent local oolong tea, grown on plantations all around. The tea bushes here was originally imported from Taiwan, well known for its high mountain gaoshan teas grown in a very similar subtropical climate. As well as tea itself, tea sets from China as well as candies, fruits, and everything else you'd need for enjoying tea Chinese style are also available.
Wang Put Tan
One of the largest producers in the area, you can see the giant golden teapot guarding their plantations in the fields below their flagship store. Various grades of oolong available, from 200 to 1000 baht per 100g, and they'll be happy to serve up a sample.
For Akha handicrafts, Yunnanese street snacks and cheap Chinese imports, check out the two markets in town.
Morning Market
Up the hill and left from the 7-Eleven, true to the name this place opens up at the crack of dawn and most of the action is over by 8 AM, making this a great place for an early breakfast. A few shops do linger open until noon and beyond.
Tourist Market
Opens in the afternoons on the south side of town, with Akha selling local produce tea, mushrooms, herbs and knickknacks to tourists.