After touching the button SWAP you will be asked to select one or more of your items to offer to the owner of the item you like. By touching each of them will be highlighted in pink , i.e. they are selected. If you touch them again you will unselect them.
Once you chose what to offer touch the SWAP button below in the page.
ATTENTION: you will be asked to confirm your offer again because by going on (by touching the OK button) you will commit yourself to send or deliver your items in case the other part will accept your offer!