Wheldrake -- why not trade with Europe while the tax rate is low, then switch to trading with the natives when the tax rate gets high enough that you can make more cash selling to natives despite the low prices?
And, later in the game when cash flow considerations and storage capacities aren't as pressing an issue, I try to store up raw goods until I can process them into finished material, since that effectively lowers your tax rate on the raw goods to 0%. Also, once you have a processing factory set up in a city with high rebel sentiment, you get that huge 50% processing bonus on top of the rebel production bonus, which is pure profit. When you can turn 12 tobacco into well over 20 cigars every turn from a single Master Tobacconist (and you have three such experts in every factory), you can generate wads of cash.
In my current game, despite having declared revolution, my specialized processing city is running full blast and shipping huge amounts of finished goods to Europe, generating thousands of gold per turn which is used to buy reinforcement militia units to ship back on the return trip. Course, it helps that I used the "save good results at the end of the turn" exploit to have just a 3% tax rate -- if the tax rate was in the 60% to 80% range, I might have shut down the factories long ago and turned the specialists into fortified soldiers (who can be converted into dragoons as needed), making the initial stages of the war easier to wage.