Stealing By Trading

[SAMisaNUB]

So a guy named Jomari asked to see my ak with s1 e1 and p1, i was gonna put p2 on it, he then told me to decline my ak, and when I did a white rectangle appeared. He gave me 10k and I couldn't equip my ak, I restarted the tab and my ak was gone. Does anyone know how this happens?