lets say you're buying a set of tubes from ebay. The government gets a cut from the sale, whatever that cut is, lets say 15 percent.this is called sales tax. the government is also taxing the seller on any profit he makes . the government calls this income tax. When you approach a seller with the offer of doing the deal under the table ( just a point blank " can you send me your email address " ), they usually agree because then they make more $. No-one likes being tracked by the government. You pay straight into his bank, and he sends you a ' gift'. Sellers have even offered to me items for less than the original sale price, as they save so much money over a period of time doing this. Once the first deal is done, well it's first name basis from that point on. "Andrew, this is Hans Gruber Sphinckell , I need some tubes what have you got?" - sent to his private email. * you don't have to pay sales tax, and he pays less income tax.