Your Filter:
'COB:Date_Purchased >= p_web.GSV('Sales_start_date') and COB:Date_Purchased >= p_web.GSV('Sales_end_date')'
I would make:
'COB:Date_Purchased >=' & p_web.GSV('Sales_start_date') & ' and COB:Date_Purchased >=' & p_web.GSV('Sales_end_date')
Hope this helps
Cheers
Vinnie