It is difficuilt to gauge how many players have '' Scored '' for both teams, as there have only been ten players who have played for both between the years of 1912 to 1997.
From records from what I can find, roughly out of the nine players that have made the switch only six players have scored for both teams, these being ;
From 1967 there has only been two players to play for both Manchester United and Liverpool, but not being sold directly. These two players were Peter Beardsley and Paul Ince, although Beardsley never scored a goal for Manchester United, only appearing in one game.
Dirk Kuyt scored three goals against Manchester United
Jermaine Pennant, Scored an own goal for united as a Liverpool player and scored against united as a Liverpool player and won the Prem with THE ARSENAL but was born in Jamaica but has represented England.
Not at Liverpool he did not. See here (http://lfchistory.net/stats_own_goals_lfc_all.asp) list of all own goals scored by Liverpool players
paul ince Michael Owen
No player has hat trick against manchester united