The Items you'll need are: Gracidea, Golden Bows and Lost Loots, Tight Belt, X-Ray Specs, Mobile Scarf, 3 Max Elixirs , 4+ Reviver Seeds, 5+ Apples. Oran Berries and Escape Orb are optional.
Now head off to Marine Resort and scout for a Kecleon shop. The bigger a shop, the more Golden Ribbons/Lost Loots you can put on the mat.
USE YOUR GRACEDIA FLOWER AS SOON AS YOU GET THERE BECAUSE WHILE YOU ARE SHAYMIN SKY YOU GET DOUBLE SPEED THE ENTIRE DUNGEON!
On every floor swap your Tight Belt for your X-Ray Specs and look for a red still dot near lots of Items. It Is most likely a Kecleon Shop. Now put your Tight Belt back on. Once you get there, use the swap choice on all the Items Kecleon has and replace them for what you're gonna sell, and make sure you fill the empty spots. Once you have the mat full of everything you're gonna sell, talk to him, but DON'T say yes to buying anything. He will only chase you If you leave the mat without saying yes. After he buys your Items they will be treated for sale. Replace those Items with the ones you couldn't place because the shop mat was full. Rinse and repeat. Now that you've done all of that, put on your Mobile Scarf and BOOK IT. Might I add you should find the stairs before you go to the shop so you know where to go. Your belly goes down In walls no matter what so when your belly Is empty, don't eat an Apple yet. Wait until your health gets slightly lower so you are giving yourself more time to get to the stairs before using an Apple. Now keeping doing the X-Ray Specs method until you find another Kecleon Shop. If you reach the end, congratulations! You've done this right! I completely tested this myself and It works 100%