Regardless of whether you are selling a Dota 2 item or just want to trade some skins, the process of doing so is fairly simple. If you are using the official Steam market, all you need to trade or sell items to another user is that both you and the user have a registered steam account. Once you have that, you are free to sell items with any user from the Steam community and exchange skins between accounts without any additional fees. Most of the transactions are instant with a high level of security and support offered by Valve. Keep in mind that in order to exchange newly acquired items, you might have to wait up to a week.
If you are looking to trade via one of the third party you will need to connect your Steam account to the page that you are using which is pretty simple to do. All you need to do is sign in with your sign-in information on the website that you are using. Once you do, all you have to do is select in your inventory that you want the buyer to have, and choose an item or a set of items that you wish to receive. Make sure that you double-check all of this information because once you confirm the trade, there is no going back. When you are satisfied with your choices and any additional fees that could apply, simply confirm the transaction. In most cases, the transaction will be instant, however, while usually fast, some transactions and additional fees may take up to a couple of days to complete.