usually people tell you to find your own mm then then when they get added to a skype chat the scammer makes a 2nd chat where he uses a 2nd acc to pretend hes the mm and then he trys to get you to give your stuff to the fake mm before you realise what happened.
generally this involves faking dcs/skype issues etc.
Super common scam in the csgo trading scene and i have seen posts here where people showed that someone tried to scam them that way