Hmm... difficult. You have to spend supplies to make the trade goods, so if you trade the trade goods back to get supplies at a 1:10 ratio, I'm pretty sure you will lose your shorts on the deal. (I.e. trading with the wholesaler would be a losing proposition... it's better just to halt the production of trade goods until you build up supplies.) If you allowed players to trade supplies with each other, most likely everyone would want to be on the receiving end, since supplies are what is needed by just about everyone... so the trades would just sit there. (Also considering you would want a LOT of supplies for your trade goods, no one would want to make that trade.)
The bottom line is that you have to balance your supply production in order to have a healthy city. My "rule of thumb" is that you should have one workshop for each factory in your city (count marble and plank as 1/2 each). (If you have way more factories than workshops... you need to delete some factories and build workshops until they balance.) You can advance steadily with only 3 of each of your boosted goods (2 if you are a young city). It is OK to have some of everything until you unlock the boosted tech, since they all have similar production. After you unlock the boost, you should only produce boosted goods if you want to be efficient. Obviously, a rule of thumb is approximate... as you progress you will learn how to tweak your city for the best results.
(Caveat: if you are playing on a new server, you DO need to build a little of everything, until trade partners can be established. On an older server, you do not need to worry about the supply of non-boosted goods.)
This gives you enough supplies to run your factories, build a moderate amount of troops, and keep your technology from bottling up. If you are a point-monger, this does not optimize your points, but it does balance your production.