there a various factors.
petrol driven car sells on good price because a person who wants a petrol driven car will pay high price....its a fact
people asking u to lower the price are not the potential buyers for ur car.
as far as the engine condition is concerned, nothing (not noticeable damage) happens to da car runing on cng if u maintain it properly i.e. change oil and filter periodically.
people looking for a cng car are more concerned with daily expenses and they are right in their own thinking