Is this the first time I've noticed it? Has this been in before? Basically, I wanted to go from Point A to Point B but wanted to leave my current ship (Type-7) on Point A. I bought a Sidewinder, upgraded the FSD and installed a fuel scoop, and noted the total cost. Once at Point B, I sold back the FSD and fuel scoop, verified that the prices for the modules were the same, and proceeded to sell the Sidewinder. Brand new cost = 32,000 Cr. and I only got 28,000 Cr. for it when I sold it back. I even refuelled and repaired the ship in case that made a difference.... it didn't. 4,000 Cr. for the trip, basically.
I'm not sure how I feel about this. I do like the idea of things depreciating. A used weapon or FSD or fuel scoop should not be bought back at the same price it was sold at. However, I see that this is not the case for modules but it is the case for ships....
At the moment, I usually "store" my excess cash as module upgrades to my ship. Buy a couple of weapon modules, upgrade the life support systems or some such, and I get the same amount of money as I've put in. Spend 2,000,000 Cr. on upgrades and get 2,000,000 Cr. when I sell those upgrades back. If module depreciation is introduced, things should get more interesting. "Do I really need that module or should I just keep my credits?"