You make a trade on a site/with a friend/or elsewhere
The real bot from the site or your friend will send you a real trade offer
Scammers will automatically detect this trade offer and decline it on your account because they have logged into it and have your Steam API key.
The scam bot will copy the name and avatar from the real bot (account)
Now the scam bot will send you a trade offer for the same game items you sent to the real bot (account)
You will open Steam Mobile and confirm the scam trade instead of the real one
What to Do If This Happens to You
Remove all unknown browser extensions related to Steam/CSGO
Change the password on your Steam account
Deactivate devices from using your Steam account here: http://store.steampowered.com/twofactor/manage. This will prevent scammers from automatically logging into your Steam account without a password.
Do not install any unpopular browser extensions related to CSGO or Steam.
When logging into websites related to Steam/CS:GO, make sure you are on the real Steam login page and ensure that the address bar shows steamcommunity.com. If the address bar is hidden, the site is fraudulent. These login pages will look like real Steam login pages, but you are actually providing your account information to a fraudulent site, and they will be able to log into your account even if you have Steam Guard enabled.
Always open trade offers from the website that sends them to you. If you receive an error message when opening it, your account may have been hacked, and the trade offer was replaced by scammers.
If the site does not provide a link to the trade offer, make sure the previous trade offer was not declined just before you received a new active trade offer.