Well, I give you this Dale, as your production goes up, you are adding value to your stocks; tobacco leaves goes to cigars, cotton to cloth, etc. This is of course an economical increase issue in your colony; as you are gaining future profit to pay for both, the tobacco leaves and the labor spent to transform these to cigars, etc (the plus-value as I know), and still make some additional money. As long as you accumulate stocks of goods (even working the land to stock primary raw materials) your economy is growing. Yes.
Nevertheless your economy indeed growing (not completely stalled), if there are things you can do to further make it grow and you aren't, you will be putting it, in balance sort of speak, in a halt. Economy is always like this.
The fact is, all the time you spent accumulating those 300 units needed to start overflowing is investment/growth time lost. Then, you start selling very little (proportional to your excess stock as you said), this is rudimentary profit for a long time, until your overflow sales matches yours every turn production. For example, I did reach a match producing and selling about 79-80 cigar units p/t, only with a stock of not less than 980 units. If I had put the money earned from selling in Europe (F2) 980 units of cigars, specially by the turns these were produced, that money would have made me reach my 79 cigars p/t much much earlier than I did (earlier professionals from Europe, earlier structures from master carpenters or by purchase, etc).
Sorry for the long post. My point is, having the money at your disposal much earlier is fundamental for your empire's growth, and defense also. It actually has an exponential factor in economical growth (discounting randomness, wars, bad investments, so on so forth). Add to this the fact that you are making 50% of the profit from overflow sales and you can't seriously say this is exploit. It is very well balanced for me. It is usable alternatively in some cases and certain stages of your colonial development, but certainly not recommended for any case scenario.
Finally !!!... you have a good point here! It didn't cross my mind until this post this interpretation in the matter; that is what all this smuggling idea is about Dale. I know this is a game and some things just are (as if real life might be any different). But if we have to give some sort of explanation to overflow sales the best interpretation I can make out is a smuggling scenario, it is not entirely perfect, and as I said in your MOD thread, I'm not arguing it was intended for sure.
Indeed, you can call it shipping from your king at a 50% fee, same 50% fee you pay for tools when buying them in the purchase button, when your colony hasn't enough to finish a build. Though once again, the king appears on screen to ask for your confirmation to ship a treasure, and he doesn't in the other two situations (overflow sales and tools purchase), which by the way can still happen under WoI (with Monarchy). If you are at war with the King he won't be shipping tools or anything for you.
Anyways, try to sell a treasure to him while in WoI and see what happens.
