Evan Schuman's StorefrontBacktalk

Online, it’s messier. When Target, Borders and ToysRUs outsourced their Web sites to Amazon a decade ago, it was easier for state-tax agencies to argue that the traditional retailers, with brick-and-mortar stores in the state, were still responsible for sales taxes. But what if a pure-play E-tailer uses a traditional retailer’s online store to sell its goods? Or what if one out-of-state E-tailer sells through another out-of-state E-tailer’s Web site—or if just part of the transaction is outsourced? Who exactly does the tax agency go after?

California hasn’t helped itself by carving out separate rules as part of the political fight over online tax collection. For example, traditional auctioneers in California are required to collect sales taxes on every sale. But eBay—which is based in California—isn’t being required to collect online sales taxes. That’s the job of the sellers themselves, according to a deal that was cut to encourage eBay to support tax collection.

In theory, that’s because eBay isn’t actually the seller—it’s just providing a mechanism for the sale, handling the money and taking a cut. There’s a usefulness in that logic: For example, it cleanly cuts the whole payment-card chain out of the tax-collection loop, so tax agencies won’t go after processors, acquiring banks, card networks and issuers to collect sales taxes.

Then again, what eBay actually does sounds a lot like what traditional auctioneers do. It also sounds pretty much like what Wal-Mart does for third-parties like Wayfair, who are the official sellers of some items on Walmart.com.

And the messiness doesn’t stop at California’s borders. In a brick-and-mortar store, sales-tax rates depend on the location of the store where the transaction is done. In most states, online sales-tax rates are based on the location the product is delivered to. But in a few, such as Illinois (which just passed its own online sales-tax collection law), the sales-tax rate is based on where the order is actually processed.

As a result (according to the Chicago Tribune), the same item sold to the same customer by different E-tailers could have different sales-tax rates—and it’s almost impossible for customers to confirm whether the sales-tax rate charged is correct. (Of course, whoever has the higher sales-tax rate will be blamed as a thief by any customer who notices the difference.)

No matter what finally happens with online sales-tax collection, there are going to be lots of theories about exactly who’s on the hook for the taxes. Fortunately, every retailer can count on one thing: The tax collectors will always think it’s you.