Mark items as sales tax inclusive (IE sales tax included in transaction amount)

My business revolves around ebay sales more than direct invoicing. When I do invoice for local sales the built-in sales tax is nice however ebay includes sales tax in all applicable sales already so it would be nice if, in addition to setting your sales tax rate, you could set your sales tax divisor to split transactions that already have sales tax.
This would help a lot at the end of the month when I calculate sales tax because right now I have to pull out a spreadsheet and add it all up manually.