Always have the engine bay detailed before selling a vehicle. Never allow a detailer to skip it. Having a good detailer is one of the most important things a used car dealer can have. I’ve been using the same detailer for many years and he is very careful and doesn’t damage anything in the engine bay. However, some detailers are not careful and sometimes get water in the spark plug chamber causing the vehicle to misfire. Always make sure any water in or around the spark plugs is blown out really good with an air compressor or else the vehicle will misfire. It’s also really important to cover up any electrical components the computer and anything else that could possibly get damaged by the water.