Bring up the profile card of the person you want to trade and click on the tab on the left that is the top one. Then, you can see which items the Jammer has for trade and then give them an offer. If the Jammer accepts your offer, you will get the item you offered for and the Jammer will get the items you offered.
To put things on trade go to your player inventory and on the upper part of it, click on the 'Trade' tab. There, you can click on an item slot and add an item to your tradelist.
Please be aware that you can offer up to 4 items and you can cancel your offerings anytime between the time that the Jammer accepts or declines. You can also put up to 12 items for trade.