I would wager very little beyond a detail cleaning is done to a "Certified Pre-Owned" car. Maybe they give it a mechanical once-over and a quick computer diagnostic scan to be sure it doesn't have any obvious problems.

Make it look as new as you can, sell it at a premium as a "certifed" car, with maybe a 2-year warranty. Profit.

Even a 5 year old car is very unlikely to have any major mechanical issues unless it has been abused.