Fix comments and unittest for market.py and style

Trying to make the right commit to  the develop branch.
@gasull I prefer [this](https://github.com/Ignat99/OpenBazaar/pull/1/files#r18075270) alternative, because the `KeyError` is raised on the first bad lookup, whether that is `['Contract']` or `['item_price']`. It also very clearly stakes out the default value assigned.