The free way to sell items is to use the "trade" command (right click on a player, or type "/trade playername"), then you right click on the item you want to sell to the other, or you click on the money icon to write numbers if you want to give money. Finish with OK. The personal shop solution: you can purchase a shop license (7 days only) to the DATS director (Hashirama I think). The more expensive way: you can buy a Tanemon/Kamemon shop in the Cash Shop. Then after you used it, the Digimon will sell the items you want while you're away.
edit: sorry I was not logged.