To receive the purchased skin, our system must identify a Steam account to which inventory the item will be sent. You need a Steam ID in “64” format, it is a sequence of 17 digits (numbers only!).
You can get a Steam ID either in the Steam client or through the Steam website.
Log in to your Steam account and click on your username in the upper right corner, in the menu that appears, select “Account Details”. You will find “Steam ID” under your account name. This number is the ID you’ll need.
Do the same for the Steam Client. Click “Steam” in the upper left corner and select “Settings”. In the window that appears, select “View account details”.
In the window that appears, you will now find the required ID: under the account name.
Copy the ID and paste into the required window when buying the skin
Remember that in accordance with the Steam’s Terms of Service, it may not be possible to exchange skins, more information can be found HERE.