If you have Builders Club, you can create and sell some of your creations (except Models, Decals, Places and Audio, which are either free or private). By default, assets are not for sale. To put a T-Shirt up for sale, first go to the Develop page on the ROBLOX site, then select the T-Shirts category. Find the T-Shirt you want to sell and click the dropdown on the right-hand side, then click Configure.
Once you've done that, scroll down to the Sell this Item section and check the box that says 'Sell this Item'. You can then select whether how many Robux to charge for the item. Once you're done, don't forget to save your changes by scrolling down and clicking the blue Save button at the bottom of the page. After that, your item is up for sale and, so long as you retain your Builders Club membership, anyone can buy it. If you lose your membership the item goes off sale until you regain it.
Keep in mind that 30% of your profits of every item sold will be given to ROBLOX.