In order to create and upload items for sale on the Roblox marketplace, you must be at least a tier-1 Builder's Club member. For example, you can create a custom shirt (or a set of pants) and then upload it to your profile; once you've done this, you're able to sell as you please. Go to https://www.roblox.com/home in your browser. If you're logged into Roblox, doing so will open the Home page. If you aren't logged into Roblox, enter your username and password in the upper-left side of the page, then click Sign In. It's at the top of the page. Click an item type (e.g., Shirts) below the "My Creations" heading. If you have many items in your selected category, scroll down until you find the one that you want to sell. It's to the right of the item. A drop-down menu will appear. This option is at the top of the drop-down menu. Doing so opens the item's page. In the "Price" text field below the "Sell this Item" heading, type in the amount of Robux for which you want to sell the item.  If this field is greyed out, you'll first need to check the "Sell this item" box directly below the "Sell this Item" heading. Roblox takes a 30 percent cut of anything you sell. This will save your changes and make your item available for sale. Whenever someone purchases your item, you'll receive 70 percent of whatever they paid.

TL;DR in Vietnamese:
Bạn cần chắc chắn rằng mình là thành viên Builder's Club. Bạn cần chắc chắn rằng mình có vật phẩm để bán. Mở website Roblox. Nhấp vào thẻ Develop (Phát triển) nằm đầu trang. Chọn hạng mục món hàng. Tìm vật phẩm mà bạn muốn bán. Nhấp vào bánh răng Settings nằm bên phải vật phẩm. Nhấp vào Configure (Cấu hình). Cuộn xuống và thiết lập giá tiền Robux. Nhấp vào Save (Lưu).