If you want to build your own brand you must sell your products on your own platform. Some would argue otherwise. I say if you are going to sell your business and the potential buyer asks you how you get sales and you reply, “Oh I sell on Teespring, they collect the money and do customer support and fulfil orders. I get paid a commission on that sale.” The potential buyer would think that you are running a business on affiliate marketing. You will not sell your business. There is no asset. The asset you have are the designs, not the customers or traffic.