What do you mean?Then you shouldn't receive any "price" for what you can sell for until after you give them a number of what you are looking to sell.
You sell certain amount of commodity for unknown amount of money and see the price after deal is closed? IMO it is complete ***.
And if you DO see price before pressing “sell” button it changes nothing, just adds unnecessary complications. Which means you need to “offer” some commodity, see price, then abort trade and repeat process at different station.
It also opens possibility to an obvious exploit: do not sell 100t at once, instead sell 1t 100 times (actually, it is the “correct way” to sell if you want to influence BGS and being this way for years).
The only solution that I can think of is making sales delayed.
You put certain amount of commodity on sale, than wait for demand recalculation tick at which time you get your money. Market shows “approximate price” calculated from current demand and your offer, which is TOTAL amount of commodity you placed at market (even if you done it in 10 runs).
Good on paper. But again requires effort to implement and comes with additional server load.
As a result, the best solution is to not fix what is not broken.