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1This sounds like you've been hit by a Trade API Scam. At this point [b]your account is possibly compromised[/b] and you need to take these steps to secure it:
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31. Deauthorize all devices (https://store.steampowered.com/twofactor/manage)
42. Generate a new Trade URL (https://steamcommunity.com/id/<your id>/tradeoffers/privacy) (It's under Trade Offers > Who can send me Trade Offers?)
53. Revoke your Steam Web API Key (https://steamcommunity.com/dev/apikey)
64. Change your passwords
75. (In the future) Do not log in to any 3rd party sites if you do not know if it's safe or not.
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9What happened:
10A Trade API scam works as follows (usually): 
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12- you logged into a fake website and (very quickly and automatically) they got your Steam API key. 
13- this allows anyone with this key to modify trades on your behalf
14- when you went to trade your items the trade was instantly modified to send your items to someone else. Sometimes this is a bot that copies the profile of who you thought you were sending it to.
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16These items are unfotunately gone and the only way to get them back is to have the scammer return them.
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18I'm sorry but it's important you secure your account quickly!