BINGO! *checks off forum thread card[1]

The reason is the poorly represented broker system. People call it bulk sales tax, but that's not quite right, rather, if you see "500 demand, price 100", that's not actually 500 units being asked for at 100.

Assume that 500 demand was the maximum demand... at that price a single unit fetches 100. But if demand were just 50 (of that 500 max) the price per unit might actually be more like 75.

To represent that, FD have made an assumption that anyone selling goods, will sell the full amount of goods in their hold[2], and so they automatically calculate a price-per-unit based on your cargo, deducted from the demand.

That is, to simplify numbers, if there was max 2 demand, with:

PPU @ 2 demand = 100

PPU @ 1 demand = 75

PPU @ 0 demand = 50

... and you brought two units to market to sell, the listed price would be 87.5 (that is, one sold at 100, another at 75). Of course, if you sold just one unit, it'd sell at 87.5, not 100, thus the caveat that FD assumes that if you sell goods, you're selling your full holdings, not a partial holding.

It's a terrible implementation, and very poorly represented/visualised in the game.

[1] This is a pretty common question.

[2] This is evidenced by the old, now-resolved exploit of selling 1t at a time, for enhanced influence/BGS effects. Selling 1t of an item 100 times had 100 times the effect on the bgs, compared to selling 100t of an item once; i.e effects were calculated per transaction.