Wow, wish I had known this a week ago. I had a well kitted out T6 but wanted to do some fighting. I thought of buying back my old Cobra, but then realised that the Asp ought to be able to trade and fight, so I bought that. Just did a straight part-ex and thought that I spent more money than I expected given some of the modules that were on board. Did a few upgrades on the Asp, but decided that it was neither good at trading or fighting and that I would be better off with my old T6 and a fully kitted out Cobra as overall they were cheaper, and better for each individual job. So again I just did a straight part-ex, and once again thought I ended up with less money than I expected - I will know for the future when I change the T6 for a T7!
On a positive note, I think my poor ship selling skills just wiped out all the bonus credits that I had made from double selling goods over the New Year bug problem, so perhaps what goes around comes around.