By attending trade shows in Texas to introduce and demonstrate dental equipment for the purpose of selling them in Texas, an out-of-state seller had the required physical presence in Texas to establish substantial nexus with the state. As a result, the taxpayer was required to collect Texas use tax on sales to Texas purchasers even if orders were received and filled outside Texas. Federal law prohibits states from imposing income tax on a taxpayer whose only contacts with a state are solicitation of sales of tangible personal property if the orders are processed and filled out of state and if the property is delivered by a common carrier; however, the prohibition does not apply to a transaction tax, such as sales and use taxes. (Decision, Hearing No. 46,628, Texas Comptroller of Public Accounts)