No, the attempted car sale was by the perp AFTER the murders, if I understand correctly. The article does not say how the 4 came to be connected with Cosmo, but he drew attention to himself when the 4 were missing and he tried to sell Meo’s car to someone else.
One of the local articles said Cosmo knew at least some of the guys from Facebook and a public hobby group online where they sell/display fancy sneakers. (Maybe killing people over their Air Jordans????)