[FEATURE] Fix style code by PEP 8 for market.py

http://legacy.python.org/dev/peps/pep-0008/#id12
http://legacy.python.org/dev/peps/pep-0008/#id14
agree w/ @dionyziz , however, that particular comment is not the best example.
the method is not implemented, and it's above a "on_listing_results", seems like whoever left that comment is leaving it there as a "TODO" kind of note, as the method isn't called "add_listing_results" ;)

anything could happen on that callback, so that comment would be very welcome by me.