Steam Guard is an extra level of security that can be applied to your Steam account. To have an individual Steam Trade Link you will have to activate the Steam Guard protection for your profile first.

However, when using Gambling, Item Selling, or Buying third-party websites you will have to locate your Steam trading URL to use those websites since they rely on automatical Steam trading systems.When using the individual Steam Trade URL, it allows you to send you an exchange offer regardless of whether the opposite trading partner is on your friend’s list or not. There are multiple ways on how to Trade on Steam whereas directly inviting your Steam trading partner to Trade is fairly the fastest and safest method. By far the easiest way is to login to your steam and then click here and you will be redirected to the page where you can find your steam trade url. As soon as the Trade offer was sent, the trading companions would then be able to decide to accept, deny, or even make a counteroffer. Trade offers consist of selecting items to exchange from your inventory as well as items you'd prefer to get from your companion's steam inventory. The Trading System is based on exchange offers between Steam users.

Steam sell multiple items at once