may i ask you for the nation u r playing?
as i mention in another thread, for some goods there is truly to much supply in the first years.
advise:
build more merchant fleets if available, to have a foot in different markets
set maritim tax to zero, makes it more likely that other nations buy
set customs at maximum, and click on the goods until 80% are supposed to sell in domestic market, both increase domestic sells
research techs giving bonus to buy and sell
a guess:
make local support to nations, offer then after good relations are given a commercial agreement, should give you bonus of sells
depending of the nation:
buy every product of what you have too much, than sell all products of the same category.
AI is dull, they deplete their stocks and than need to buy from you. AI is as eager to get rid of products for private capital as you certainly are
and, the most difficult one:
be patient until world population and industry is developed enough that they really NEED TO BUY FROM YOU
PS:
not to forget, you can STOP production of a certain good at all, which would also make capital free for other decisions...