charging tax in Classified Ads??

I have seen quite a few ads recently from no feedback users selling all sorts of gear, many of it tubes, asking for XX% of tax if you reside in certain states...nowhere in their ad does it state they are a business/dealer/reseller. what do you do?, ask for a tax ID#?..ive noticed this happens a bit on ebay, id like to know if the extra 8% is going to the proper hands.