I've bought quite a few coins from him and some coins seem to have been soaked in olive oil or waxed. Don't know if he does it or the previous owners. My cats endorse him anyway.
Many bronzes are at least partially darkened in one way or the other. One of the common ways is with a sugary syrup which is baked onto the coin. If done right it will not leave any stickiness or discernible smell behind. Another method is using a mixture of powdered sulphur and Vaseline or some other type of thick oil. Often cleaners will use a tiny amount of this on a toothpick to darken any scratches during cleaning and wash it off when done or even coat the surface of a coin with this mixture during mechanical cleaning so that it lubricates the surface and darkens any scratches thay might be made.