copy data to the bottom after data already exist infrequentl

Leopard

In the modified code below and file attached, I saved a new (static) variable lRowLast which allows the "existing data" added to worksheet "Table 1" to be cleared before data is copied from your worksheet PURCHASE. 

Rightly or wrongly. I also assumed that the Quantity from worksheet PURCHASE should be in column D (and changed the copy lines to suit, changes in bold below).

Sub copyItems()
  Dim lRow1 As Long, lRow2 As Long
  Dim Purchase As Worksheet
  Static lRowLast As Long ' save last row in Table 1

    With Worksheets("Table 1")
        If lRowLast = 0 Then
            ' if lRowLast not set, calculate it
            lRowLast = .Cells(.Rows.Count, 1).End(xlUp).Row
            Else
            ' if set, clear data from last row down
            .Range(.Cells(lRowLast + 1, 1), .Cells(.Cells(Rows.Count, 1).End(xlUp).Row, 4)).ClearContents
        End If
    End With

    Set Purchase = Worksheets("PURCHASE")
    lRow2 = Purchase.Cells(Rows.Count, 1).End(xlUp).Row

    With Worksheets("Table 1")
      lRow1 = .Cells(.Rows.Count, 1).End(xlUp).Row
      For i = 2 To lRow2
        .Cells(lRow1 + i - 1, 1).Value = .Cells(lRow1 + i - 2, 1).Value + 1
        .Cells(lRow1 + i - 1, 2).Value = Purchase.Cells(i, 2).Value
        'copy Quantity to column D
        .Cells(lRow1 + i - 1, 4).Value = Purchase.Cells(i, 3).Value
      Next
    End With

End Sub

This prevents the repeat copying you talked about and allows modified data from PURCHASE to be copied to Table 1. It does not solve the problem of changes made directly to Table 1 (where you'd need to work out if rows have been deleted before lRowLast). You'd need to think carefully about the possibility of that.

Hope this helps.