First rookie mistake you made was showing the car with a fault. Any fault, however small should be rectified before you ever advertise or show to a potential customer.
Another mistake is telling the new customer what parts have been fitted.
He has lost confidence in the car as it looks like it’s has/had numerous issues in a short space of time.
My advice is pop round ASAP to his house. Be polite and co-operative. Do a diagnostic scan and if it’s a small issue, fix it quickly and you might regain his trust.
Otherwise refund, move on and always remember they’re called BM Trouble You for a reason.