There is a plastic cover that just covers the valve cover and the ignition coils. It is held by two cap nuts and two plastic retainers, although the plastic retainers are easily broken and usually not replaced. Under that is the valve cover, which is made of aluminum. It does cost more to remove, there are screws all around the edge and the valve cover gasket usually has to be replaced.
Under that is the head and that is way more expensive to take off. I am only talking about the valve cover. If you have a little oil on top of the valve cover, then the cover gasket needs to be replaced anyway. While the valve cover is off, your mechanic can check the valve clearance for you as well as the oil drain passages.
Now about that dealer, he cannot claim ignorance if your state requires a clear smog check before sale. Just because the CEL is off, that does not constitute a check. A code reader has to be hooked up and ALL ready lights have to be on. When a CEL is turned off and all the codes erased, all the ready lights are turned off. It usually takes a couple of drive cycles to get them all back on, only if there are no problems.
If he is a dealer and he is selling the car at his property as a private sale, that can be against the law in almost every state. A dealer or salesman can sell a car as a private sale, but only if it is their personal car. And there is a limit to that. In my state, if they sell more than 5 cars per year from their yard as private sales, then they have to have a separate business license apart from the dealership or they pay a huge fine if caught. The car’s title also has to be in their name prior to the sale.
Sometimes a salesman will see or make a particularly good deal on a trade and immediately buy the trade from the dealer, then take it home and resell it as a private sale. That is legal to do up to 5 times in a year and not all dealers will allow their salesmen to do this. If that is what happened here, the dealer where the trade occurred is still responsible for a clear smog check if required by the state.