Online retailers call Internet sales tax a 'nightmare'

> There are maybe 7,500 businesses that would be affected by the law, according to a study commissioned by Amazon, which has voiced strong support for the bill.

Keeping in mind that Amazon's support for the bill may bias them to undercount, does that number smell right? What % of online sales come from the top 7,500? Is that enough businesses to support the middlemen we are assured will pop up to help handle every state's software / jurisdictions / details on local tax rates for different types of items and shipping and handling and everything else?

Something I don't get - this seems like interstate commerce - isn't it under federal regulation and not state?