You may trade with another player on the same screen. Simply click on the player and choose the trade option. A request will be sent to notify that player to do the same when ready. A box will pop up, showing the trade. Click on items in your inventory to make offers. Up to six items from each player can be traded at once. To trade more that six requires multiple trades. It is best to move to a screen without danger, or both players wear Player Hats, before trading, as a monster attacking during a trade can be disastrous.
The Player Store, located at D6, one screen south of the Player house, allows you to sell items. You may buy them from others, one at a time, at the sign on the same screen. Some items can be bought in groups if you bring enough gold. The nearby Flower may be picked to buy "free" items.
This store may also be used to store many of the same item, by selecting the red X as the price. The store won't sell items marked this way.
The Player Store can hold a maximum of 99 items, including those sold as well as those received in payment.