IMHO - you should only be able lower the price to 10-15% below market average or 25-50% above market average.
The problem there is that the existing variance on price on a lot of goods - especially the ones with cheaper averages - is well outside that range.
Wine - market average 484 credits - can be bought for 126 credits (75% below), and sold in the right BGS states for at least 7234 credits (almost 1400% above, which exceeds even the existing Fleet Carrier limits)
(And buying/selling at silly prices is one of the few practical ways for a friend to help you fund your carrier at the moment, too, for non-BGS issues)
Some mitigations that could be used, none of which seem sufficient:
- make the BGS effect of selling goods that a station
supplies zero. That at least stops things like laundering a station's own H-Fuel (or other goods) back to it and means you have to buy the goods from elsewhere. With a 20 minute jump time and a multi-kiloton hold, this doesn't mitigate much, but it does at least stop some of the sillier options.
- put an extremely sharp diminishing returns curve on selling goods for a loss - so someone messing up and dropping their 100t Geological Equipment at a 200cr/tonne loss can hurt a faction slightly, but someone putting 200 million of loss onto a faction still only hurts it slightly. Feels like this might be necessary ... but
has to be viewed as a stop-gap measure pending introduction of proper targetable ways to attack a faction economically or militarily, or we just continue down the slope of only positive BGS actions being possible, and negative states only usually arising as side effects of Event States.
- prevent carriers setting a sell price for their goods higher than the maximum buy price in the local system. Could still be used for attacks in the right circumstances - a secondary market in a different state, or a nearby system in a different state which can at least be used to get lots of overpriced goods, but wouldn't be universally applicable.