California, say, cannot force a merchant in another state to collect California sales tax on goods it sends to Californians unless the firm has a physical presence there, like a warehouse or sales office. A state can require residents who do business with such a merchant to pay sales tax directly to the state. States sleuth through delivery or other records to uncover transactions and levy tax on buyers only sporadically, and mostly for big-ticket items. Some direct sellers refuse to collect tax, while others routinely collect it. Dell only for sales in Kentucky, Nevada, and Texas.
Smaller firms are the most likely to take a narrow view.
Quantex Microsystems adds tax only on sales in its home state of New Jersey. MicroWarehouse on sales to Connecticut, Illinois, New Jersey, and Ohio. The issue of sales tax in cyberspace is not limited to computers. Existing escape hatches on mail-order sales tax may narrow as more firms feel pressure from tax collectors. You may be able to evade sales tax for now, but it's a ploy that's likely to get more difficult.