This video has all the locations of every trading sequence for the trading quest in The Legend of Zelda: Link's Awakening DX. Read the description for more indepth information.

Trading Sequence #1 (0:00):
Trendy Game in Mabe Village. You win the first trade item from this minigame.

Trading Sequence #2 (0:50):
Papahl's wife in Mabe Village. Give your Yoshi Doll to her to receive a ribbon.

Trading Sequence #3 (1:18):
Mini BowWow in Mabe Village. Go into the kennel right beside Madam MeowMeow's house and give the BowWow the ribbon for some dog food.

Trading Sequence #4 (1:49):
Sale on Toronbo Shores. No, not an actual sale, that's his name. Give this talking alligator the dog food for some bananas.

Trading Sequence #5 (2:26):
Kiki near Kanalet Castle. Give Kiki your bananas and he'll summon a horde of monkeys to build a bridge for you, along with conveniently leaving a stick behind...

Trading Sequence #6 (3:09):
Tarin in Ukuku Prairie. Once you beat Key Cavern, head back to the tree with a bee nest to find Tarin trying to get some honey. Give him the stick so he can get stung and you can take the honeycomb for free.

Trading Sequence #7 (3:48):
Chef Bear in Animal Village. Give this friendly bear the honeycomb for a pineapple.

Trading Sequence #8 (4:13):
Papahl on Tal Tal Heights. If you talked to him at the beginning of the game, he did mention he would get lost, and loandbehold. Either way, give him the pineapple so he can revitalize to get the hibiscus.

Trading Sequence #9 (5:01):
Christine in Animal Village. Head to the house on the left in the most northeast screen of Animal Village to meet this talking goat and get her love letter.

Trading Sequence #10 (5:37):
Mr. Write in Goponga Swamp. Give him Princess Peach's letter to get... a broom.

Trading Sequence #11 (6:18):
Grandma Ulrira in Mabe Village/Animal Village. Depending on when you do this, she could be in either or. Talk to her and give her a new broom so she can continue sweeping to keep the island nice and clean, along with getting a nice fishing hook.

Trading Sequence #12 (6:52):
Fisherman under the bridge in Martha's Bay. Give the fisherman this hook and you'll be awarded whatever his next catch is, which happens to be the mermaid's necklace (or bra if you play the Japanese or other specific international versions).

Trading Sequence #13 (7:40):
Mermaid in Martha's Bay. Give the mermaid her lost item back to get a scale from her tail.

Trading Sequence #14 (8:13):
Mermaid statue near Martha's Bay. Insert your mermaid scale into this statue to reveal a staircase that leads to the final item that ends this whole trading quest: Magnifying Lens.