Sales tax has nothing to do with paying employees, overhead, size of the business, etc. We as businesses collect it only as a pass-through to the state. I live and work in Nevada, but my firm does a lot of work in California, Arizona, New Jersey, Mississippi, Oklahoma, Texas, and a few others. I collect sales tax from my customers (mainly casinos and developers, but some individuals) and at the end of the month I print a report detailing what I've collected from whom and pay the various states their share. I make no money on the tax since my business checking pays no interest, and my costs are a small amount of administration. So in essence it costs a business some small amount to comply with the tax laws. It's just a cost of doing business.

Now if someone on eBay has a legitimate business that is licensed or not, they may be required by their state to collect sales tax. In Nevada you cannot collect and pay sales tax without a license, in California you can. Whether or not the business actually files the form and pays the tax is another matter all together (and not typically one the IRS cares about since they do not enforce sales tax laws). Many businesses collect the tax and pay either none or a reduced amount to the state. The state can and will periodically audit you...if they know you are a business...

Just my couple of pennies...