Welcome to the motor trade!
What I tend to do is:
Buy low mileage cars
Buy or retail nothing over 9 years old, unless its a Japanese or Korean car
Never buy trouble (known faults)
Drive and scrutinise everything before it's advertised
Do a multipoint check on paper so you know everything functions
And with all that stuff WILL still go wrong. Squeaks, rattles, vibrations are what annoys me the most. They can be hard to diagnose. An EGR of flywheel can be diagnosed over the phone.
Warranty issues tend to come in cycles and usually on a Monday morning before 10am after you've had a hard weekend down the pub
Good luck!