I'd not have bought a car that was not properly maintained. I've heard of dealers who didn't even change the o-rings when changing oil/filter.
I'd be more impressed if you told me, "he did all the work himself..." (Did you miss my red, bold, italic sigline?????????????????)

Just remember; Misfires can be caused by:

Bad spark (common)
Bad fuel
Low fuel pressure
Fouled injectors
Major issues with the ECU (not at all common)
Filthy/impure electrical power to the ECU (Causes: Alternator with corrosion on the field rings, really bad battery, open or resistive battery cables/ground straps somewhere, etc.)

If the battery is more than four years old, or if the car really did sit for a LONG time - go get a new battery in the morning.
If you respond with "I had the battery tested", I will never respond to one of your posts again!