I don't know if this happens for all resources, or just spice:
Selling a stack of MTIs in another MTI port, you are paid as if you sold 1 single unit instead of 40-120+. So, screen says value: 1700, profit: 340, but if you sell 10 or all units at a time, it treats the sale as a single unit of goods sold. Lost 120k that way.
More detail: Playing main campaign as England, I take over Bombay with the 5 mission ships you get and build an upgraded trade outpost (to 15% discount). When I buy spice in Bombay, 40 tons worth, I sail over to the Porcelain port, where I find that they'll buy Spice at 1700/ea, a "profit" of 340 according to info screen. So I drag and drop the whole 40 stack and get paid a *total* of 1700. For all 40 units of Spice. Ouch. Had to replay an entire year to get that back.
Selling a stack of MTIs in another MTI port, you are paid as if you sold 1 single unit instead of 40-120+. So, screen says value: 1700, profit: 340, but if you sell 10 or all units at a time, it treats the sale as a single unit of goods sold. Lost 120k that way.
More detail: Playing main campaign as England, I take over Bombay with the 5 mission ships you get and build an upgraded trade outpost (to 15% discount). When I buy spice in Bombay, 40 tons worth, I sail over to the Porcelain port, where I find that they'll buy Spice at 1700/ea, a "profit" of 340 according to info screen. So I drag and drop the whole 40 stack and get paid a *total* of 1700. For all 40 units of Spice. Ouch. Had to replay an entire year to get that back.