Don't be tempted to sell all your new equipment as soon as you get it. When you unlock a new tank slot, having powerful equipment on hand will let it hit the ground running.
Quick Money
You can earn money quickly by selling unwanted equipment.
Upgrading Tanks
When a tank is being upgraded you will not be able to use it so it makes sense to perform any heavy upgrades after you have used all your fuel as you will be unable to use it for a while anyway.
Various Tips and Tricks
Think about a need to replay previous levels to earn bolts and sell unwanted equipment to earn quick money.
But don't go and sell all new equipment as soon as you get it. Once you unlock a new tank slot, having powerful equipment on hand will let it hit the ground running.
You can create new parts by using the money you win from multiplayer games or by spending real money. You can get some great equipment either way and it makes multiplayer worth playing.