Forbes, Social Confusion: How Do Sales Taxes Apply to a Groupon?, by Janet Novack:

Have you used a Groupon or a Living Social coupon recently?

When you handed over your coupon, did the merchant collect tax from you? Did he calculate the tax based on the full face value of your purchase, or the discounted amount you paid for it? Did he force you to ante up any tax in cash, or did he allow you to apply your coupon towards the full bill,  including tax?

These aren’t academic questions–the answers could affect the attractiveness of Internet-based social coupon programs to consumers, merchants, and state tax collectors.  Ultimately, the question of how social coupons should be taxed is likely to end up in court. …

In case you’ve missed this latest craze, in a typical deal you might pay $50 with your credit card to Living Social or Groupon over the Internet, and get via email a link to a coupon worth $100 at a  local shop. (The social part comes in because in some cases the deal only goes through if a certain number of folks buy it. Plus, there are often incentives for referring your friends.) Neither coupon site collects taxes when you make your purchase and they warn their vouchers can’t be used for sales taxes or tips—unless the merchant allows otherwise. Groupon states in its Merchant Self-Service Agreement that the merchant “shall be responsible for paying all sales and use taxes related to the goods and services described in the offer.”

But just what are those?  Spokesmen for tax administrators in three of the nation’s five most populous states—California, Florida and Illinois– told Forbes they expect merchants to collect sales tax on the face value of what you buy. In other words, they want their cut on $100 even though you only paid $50 and the local merchant collected maybe $25.  By contrast, if a store printed up its own half-off coupon and allowed you to buy an $100 item for $50, the tax men would only tax the $50 you forked over. …

There’s a nice irony here: If the states can legally charge sales tax on the full face value of the Groupon, this could be a rare  case in which they get a windfall (as opposed to losing gobs of tax revenue) from e-commerce. They lose out from Internet shopping because web powerhouse Amazon.com, among others, doesn’t’ collect sales taxes from the residents of most states.