Commodity sell price too low

This initially does what I want to fix #3152: same buy/sell price for commodities, fuelclub, and goodstrader, so only ship equipment in EquipmentMarket currently have the 20% price reduction.

It would probably be saner to then have EquipmentMarket as a special case, rather than everyone else (CommodityMarket, GoodsTrader, FuelClub), I can fix that, just say the word.