Currently the in-game metric for profit for selling cargo is a bit bugged out. When you see a +80 profit next to an item in the market, it will only give you that +80 once for the full sale. For whatever reason, even if you sell 10 items at +80 profit at once, only +80 is added - there is no 'multiplier'.

The way around this is to sell one item at a time. So click right arrow, then complete sale. No need to exit the market screen, just repeat. One by one. So if you find that your OIL is +150, then selling 100 units individually should raise this metric by +15000 overall.

This should count towards the Capitalist achievement (I think) where you earn 25000 gold by selling cargo at markets. My achievement only popped up after I had completed all the in-game bonuses for market selling, which is +35000 max. The achievement popped up some random time after that.