To trade items on Roblox, start by going to roblox.com and enrolling as a member of the Builders Club for a monthly or an annual fee. Next, collect items to trade or invest in the game’s currency of Robux. Make sure that your account settings reflect that you are open to trade.
The accounts, activities, items, games and models are for you to play with while on the site. Except as permitted by the functionality of this site, you can’t sell them (for money or ROBLOX Points), give them to anyone, trade them for anything (including ROBLOX Points), or pretend you made them.
The only items you can trade are as follows:
Click the gear to the right of the item you want to sell. Click Configure, then select Sales. On the Sales page, click to turn on the "Item for Sale" option. Enter the amount of Robux for which you want to sell your clothing. Click the Save button.
Clothing items cannot be sold for free, the minimum amount for Shirts / Pants is 5 Robux and for T-Shirts is 2 Robux.