Don't Sell Old Ships To Afford The Next One
A mistake CMDRs often make is selling that hard earned workhorse, to buy the next one, a good example is people selling a Python (56~mil credits stock) to afford the Anaconda (120mil credits stock), or selling the Vulture to afford that sweet FDL.
Don't Do It!
First reason, is you may not like your new ship, how it handles, its limitations, etc, so you may want to go back to your old one. Which would mean rebuying it all.
Second reason, if you die one too many times, and you're low on cash, you have nothing to fall back on. And if you die without enough for a rebuy, and no spare ships, it's back to the free stock sidewinder you go. Cheaper ships have lower rebuys, so if you're struggling to upkeep your next ship, take your old one out to earn cash with minimal risk.
Third Reason, is if you can barely afford the ship, you won't be able to afford to outfit it correctly. If it's not outfitted correctly, it'll be less effective than your old ship. A well equipped Python is far more survivable than a poorly equipped Anaconda.
That is all.
CMDR Cosmic Spacehead