I just installed the Sell Equipment OXP.
According to the readme I should be getting about 60% of the original value for the modules when I sell them, however all sellable equipment gives me just 10 credits...
I noticed the following in the script.js of the Sell Equipment OXP:
I think that the 10 credits you see displayed is the handling / brokerage / whatever fee it costs you to actually sell specific equipment. This works in a similar way to 'Unmount and sell all pylon mounted weapons' in vanilla Oolite.
Try that one with a fresh Jameson to see what I mean: it costs you 20 credits to sell missiles, the value of the 3 unused missiles comes to 90 credits, so you're 70 credits better off if you sell the missiles. As if by magic!