Money/Part Bug:
When you spend all your money on any one part, the game will sometimes glitch and refuse to set your money to zero, and instead, deduct no money at all while giving you the part in question.
Bargain A Part:
When you “check the price” of a used part (but not buy it), you can back out the used parts store menu and re-enter. Sometimes (not always) the parts may have reduced in value.