They do not sell a lot of trinkets in Guinea, but they do have wonderful clothing that you can purchase. The tailors there are very skilled and can create an outfit very fast approximately within a week. Masks, wood statues, djembes drums, traditional clothing, bags made in Guinea are sold in many of the areas outside of major hotels in Conakry and along the roadside. Always barter, especially if outside a major hotel as prices there are higher. A good rule of thumb is to halve whatever the opening price is and also to walk away if the prices don't come down. Negotiations are supposed to take awhile and are a way of figuring out the "walk away" price point for both buyer and seller.
The largest market in Conakry is Madina market. You can find everything and anything there. Be careful of pickpockets, mud during rainy season and traffic. Its a pretty hectic and chaotic place but you'll find the best produce, electronics etc at the best prices. You can hire a young boy to haul out your purchases for you if you are walking back to a parked car or where you're staying. Cost is about GNF5000 USD1.
In certain parts of the country you can also find some nice carvings, many of which are created in the city of Kindia.