The checkout can only read your items if your Steam inventory is public. If it says it found no tradable items, check this first.
Make it public
- On Steam, open your profile and choose Edit Profile.
- Go to Privacy Settings.
- Set My profile to Public, and Inventory to Public.
- Changes save on their own. Go back to the checkout and press Reload.
You can make it private again after your payment is complete.
Still no items?
- Wrong game: the checkout loads one game. Check that your items are for the game shown.
- Items are not tradable yet: new purchases from the Steam store or market, and items received in a recent trade, can be locked by Steam for some days. They show up once they become tradable.
- Your account cannot trade: Steam limits trading on accounts without Steam Guard, with a trade ban, or after a recent password or device change.
- Steam is slow: Steam sometimes takes a moment to show changes. Wait a minute and reload.
If your items load but some cannot be picked, see Why some items are not accepted.