Online purchases from sites like Amazon.com and eBay may seem to arrive in a state of untaxed bliss. But the law actually requires shoppers to pay their own state's sales tax rate--the concept is called a "use tax"--and voluntarily cough up the exact amount owed each year at tax time. Tax bureaucrats for years have lamented the difficulty of collecting use taxes on catalog and mail order sales. Now, with online shopping growing rapidly and nearing $100 billion a year in consumer sales, tax collectors are <A HREF="http://news.zdnet.com/2100-9588_22-6060450.html?tag=zdnn.alert">adopting more aggressive tactics.</A>