Just a quick guide on flipping junk cars quick. This is a great way to bang out the Achievements "Gambler and Scavenger." You'll need the upgrade "Welder" and Car Wash. Optional upgrades that increase the amount of money you'll get/reduce money spent are Regular Customer 1-3, Good Judgement, Body Repair Station, and Renovator 6.

Simply go to either the Junkyard or the Auction (junk cars). Repair or replace any body parts and windows, use the welder to fix the frame, and use the Interior Detailing kit. Don't bother replacing any parts or engines, these are quick flips.

Painting the car does not increase its sell value. Washing the exterior of the car does not increase its sell value.

You should be able to quickly flip cars for between 5-15k profit doing this. Have some cash saved up before doing this. Not much, at least 20k just to start. The vehicles from the junkyard/scrap auction should only cost between 2k-10k. Welding costs a flat 500, washing is free, and repairing is much cheaper than buying new parts. New parts overall shouldn't cost more than 2k.

If you do the Barn, you're looking at a full rebuild, which takes forever. Barn cars routinely cost 20k+, and can even cost more than the're worth as a flip.