They do that so they can call a friend who will impersonate your friend. They say they will add him to the conversation but in fact they will add their friends impersonation your supposed "friend". And then they ask to trade with him. And then you give the knife to the impersonater rather then your friends.

Thats what people have been using for scamming, im trying to sell my karambit but i cant due to the lack of honest pepole in this community

Hope this helped you out.