The StartPoint and EndPoint of a line returns an array of the three coordinate values ... i.e. X,Y,Z So...since you are wanting to determine the left and right of the line, use the X (0th element of array) to compare...
Similar to
PT1 = Lijn.StartPoint
PT2 = Lijn.EndPoint
If PT1(0) > PT2(0) Then
Lijn.StartPoint = PT2
Lijn.EndPoint = PT1
End If
To delete an existing selection set that you know the name of, but do not have a reference object
ThisDrawing.SelectionSets.Item("Line").Delete