Tax Bite Imperils Amazon

That means even a giant such as rival eBay(EBAY Quote), which sees many small sellers list their products in its platform, could gain a relative advantage. "That's why we have said there should be a fair application of this collection requirement," says Misener. "If people are truly concerned about fire engines and schools at a local level, a company like eBay should not be able to hide behind these kinds of exceptions."

Still, Misener says that despite progress by participating states, tax laws remain too unwieldy, and he doesn't expect the bill to pass. And beyond any partisan considerations, the bill may be held in check because neither party would want to pass legislation that seems like a tax increase before the presidential election next year.

"It could be another five or six years before anything really happens," he says.

But for investors in a stock such as Amazon, which trades at six-year highs, the impending legislation should be enough to give pause.