The “internet sales tax” approved yesterday by the Senate won’t likely stall the overall growth the online retail economy. The migration of commerce to the web and mobile devices is about much more than the tax-free percentage shaved off in online shopping carts.

But the transition to an omnichannel sales-tax system will hardly be seamless, at least according to opponents of the so-called Marketplace Fairness Act. In this huge new experiment in regulating interstate commerce, they say small online businesses will wind up playing the guinea pigs.

That’s exactly what the big retailers supporting the measure want, says Brian Bieron, senior director of global public policy at eBay Inc.

“They’re businesses. They want to put a tax cost on their little competitors,” he told Wired before the final vote.

Supporters of the bill, which allows states to demand out-of-state online retailers collect sales tax on their behalf from in-state customers, include the biggest names in U.S. retail: Walmart, Target, Amazon, Best Buy. Except for Amazon, each of these companies is a huge brick-and-mortar retailer. All four are also huge online retailers that would be forced under the bill to start collecting sales tax as demanded by individual states.

Bieron argues that the biggest online sellers benefit from the disproportionate cost he says small online retailers would have to take on to meet the measure’s requirements. That cost, however, won’t come from lost business due to higher prices, he says, or from having to install software that calculates the different tax rates depending on where online orders will be shipped. (The bill requires states to provide free sales tax software to businesses.) Rather, he says, the pain smaller businesses stand to suffer will come in the form of lost time and attorneys’ fees from having to deal with the bureaucracies of the 46 states that collect sales tax—pain that the biggest retailers have the human and financial resources to withstand.

“The complexity is not first of all in the calculating of the tax,” Bieron says. “The reason this is the challenge to small businesses in particular is the number of state tax authorities who can come after you with their tax laws. … That’s a dramatic and negative change. And there’s no software for that.”

If the House also passes the bill and the president adds his signature, which his spokesman says he’d do, businesses would have at least 90 days to prepare for the change, depending on how quickly each state rushes to get its share of an estimated $23 billion in lost sales tax revenue.

And the bill does contain provisions clearly intended to lessen the pain for small businesses. Along with the free software, states must each set up a single agency responsible for managing all sales tax issues related to what the measure calls “remote sales.” In other words, businesses will not be forced to deal separately with cities that have their own sales tax rates.

The law, which applies to any online business that does more than $1 million in annual sales, would also exempt those sellers from any liability due to screw-ups by the software or the state.

But it’s that question of who screwed up that seems most likely to keep the federal court system busy under the Marketplace Fairness Act. And it’s the Walmarts and the Amazons of the retail world that would be best equipped to bear the costs of those kinds of conflicts.

Supporters of the bill argue that eBay is mainly out to protect big sellers on the site who exceed the bill’s $1 million threshold. After the vote, Bieron said eBay would continue to seek a version of the bill that exempts any business with less than $10 million in annual sales or fewer than 50 employees.

In a statement, the Marketplace Fairness Coalition accused eBay of disingenuousness: “eBay’s opposition to the Marketplace Fairness Act is a transparent attempt to maintain an unlevel playing field and benefit a very small number of their largest sellers. It is long past time to establish a fair and level playing field on which all sellers can compete.”

The “level playing field” argument propelled the bill to success in the Senate speaks to how far online retail has come in fewer than 20 years. Shopping online isn’t something special or obscure that’s limited to a small group of the tech-inclined. It’s just another medium for buying things. Shouldn’t online retailers have the same obligations under the law as anyone who sells out of a physical storefront?

At the same time, the very idea of a level playing field implies that online and offline retail are two separate teams—an increasingly false dichotomy that exists mainly in the sentimental rhetoric of politicians and lobbyists.

“Make no mistake; the current bill addresses concerns for small businesses both online and on Main Street,” the Marketplace Fairness Coalition said in its statement, as if the owner of the general store down by the soda fountain hadn’t heard of this newfangled thing called the internet.

In reality, Main Street retailers are also online retailers, or at least they should be. The big-name supporters of the internet sales tax know this, as evidenced by their heavy promotion of their apps and websites. The online economy is the economy. Any business big or small that doesn’t acknowledge that reality won’t fare well over the next decade—no matter how much sales tax Amazon is forced to collect.