Try this:
Private Sub Worksheet_Change(ByVal Target As Range)
If Range("A" & Target.Row) = Range("B" & Target.Row) Then
MsgBox "Buy - " & Range("E" & Target.Row)
ElseIf Range("A" & Target.Row) = Range("C" & Target.Row) Then
MsgBox "Sell - " & Range("E" & Target.Row)
End If
End Sub
The key is using the Worksheet_Change
event in the worksheet where you have your data.