As far as I am aware, the Yuan (essentially a unit quantity of currency) is fully convertible and can be traded, but the Renminbi (the currency itself) is not. It's a bit weird that these are not the same thing. It may not be possible to use the Yuan for everything you need without also moving some Renminbi around. However, you can use the Yuan as the unit on which the trade balance of a country is counted - $1 billion Yuan of oil against $1 billion of silicon chip for example - without any Renminbi changing hands.
The point of US dollars is that there are more people who accept US dollars for their trades, and therefore settling in US dollars is safe - you will always find someone else to trade with holding US dollars.
Barter, on the other hand, even if using yuan as the unit of accounting, has the drawback that you can only trade with people who accept yuan and is willing to barter with you (or you find a willing set of trade partners to chain barter with).
Transferring and settling balances are different. You can also transfer Yuan without settling in Renminbi. This is very confusing to deal with, but think about transferring balances as bank account numbers and settling as moving cash. You can do a lot more than just barter without using cash.
