On Saturday, Amazon.com began collecting sales taxes for the first time on Internet purchases made by California residents. The move, which Amazon battled for years, represented a major victory for brick-and-mortar retailers, particularly small, independent stores who correctly argued that Amazon’s sales-tax exemption had awarded the online giant an unfair competitive advantage. However, there’s a large loophole in the sales-tax deal that is allowing a substantial portion of products sold on Amazon.com to remain tax-free.