when starting car, engine warning light on odb shows P0302, U0146, B2784

You didn't mention the mileage on your car but if your car has over 150k miles on it, the misfire on cylinder 2 may just lead to a failing head gasket. This failure is probably the main cause of people selling these cars to unsuspecting buyers

 

And Previous owner most likely disconnected the12v battery wiping all the codes before selling.

 

If you're looking at a car to buy and you have a scan tool, a great way to check for that is to ask it the status of the emissions monitors.

When a code clear is done, or the codes are cleared by unhooking the battery, all of the monitors are set back to 'incomplete'. They won't go back to 'complete' until the car has been driven enough for all of its self-tests to run and for codes to be set for any issues that are detected.

That feature was built in to foil people clearing their codes just before going in for an inspection, but it works just as well to indicate a code clear just before you looked at their car for sale.