Generally, anyone engaging in the business of buying, selling, or exchanging motor vehicles of any kind in the state of Texas must get a Texas Dealer License. 

You are able to sell up to 5 vehicles a year as long as those vehicles are titled in your name and used primarily for personal, family, or household use. If you wish to flip cars for a profit, you will need a Texas Dealer License.

Not sure if you need a license? Check out this post on selling cars without a license.