Private Sub CommandButton1_Click()
    Const sDELIM As String = ", "
    Dim rCell As Range
    Dim sMergeStr As String
    With Selection
        For Each rCell In .Cells
            sMergeStr = sMergeStr & sDELIM & rCell.Text
            Next rCell
            Application.DisplayAlerts = False
            .Merge Across:=False
            Application.DisplayAlerts = True
            .Item(1).Value = Mid(sMergeStr, 1 + Len(sDELIM))
    End With
End Sub