WIP: Enable different sell/buy for ship equipment

This is to fix #2953

It's the same as #2787, except I've excluded the goods trader and commodity prices (since trading is way to difficult/broken as it is), so just affect the ship equipment prices, so the space station buys the equipment back for 0.8 of the full value.

__NOTE:__ I haven't tested this yet, so I wont merge utill I've done so, sometime after the weekend. Possibly then I can dust off my "second hand" module, which allows player to buy used ship equipment at a reduced price (but higher than 0.8) from the BBS.