If the marketplaces can charge and remit the sales tax on all the sales automatically for the sellers it won't be such a big deal. However, if each seller has to, there it will cause a lot of problems and expect nearly every small seller / business in retail to close down and stop using eBay or Amazon to sell on, as keeping track of and filing sales tax in every state and muinicipality will be too overwhelming, expensi ve, and burdensome for most sellers.. Sellers flooding the US marketplaces likely would benefit, as t hey the US tax laws can't touch themand likely will never send the funds in to each state. It's something the marketplaces will really need to handle.
