When you buy an item on Redline.gg page, the item is sent directly to your offline trade offers on steam. They are sent to the account that you are logged in.
First of all, check that the Steam trade URL is correct and that it belongs to the account you are logged in with, this is very important, because you can't withdraw your items if the trade URL is mistaken or doesn't match with the user logged in the Redline.gg website.
Also your Steam inventory settings must be set to public to recieve offers. You should not have any trade bans active on the account.
You can go to "Pending rewards" and, if available, click the "Withdraw" option. After that you will recieve a Steam offline trade offer with your rewards in the account that you are logged in.
Also make sure that you are not limited on trades by steam, you can see the it here
If none of the above work, please contact us.