To trade on PlayerAuctions, you will need to set your Steam inventory to public. To do that, go on Steam, click on your username in the top bar, click on "Edit my Profile" on the right, and then on "My Privacy Settings". There, make sure that your inventory is set to "Public".
Your account will also need to have the Steam Guard Mobile Authenticator activated for more than 7 days. If it's not the case, every trade you make will be put on a 15-day hold by Steam. PlayerAuctions will not be responsible if you have to wait a few days to get your item. Make sure to set up the Mobile Authenticator beforehand!