Fix comments and unittest for market.py and style

Trying to make the right commit to  the develop branch.
The standards exist to increase code readability. Your changes decrease readability, because they make the code look like there's a real if whereas it is only an inline if. Since you are breaking long lines into multiple smaller lines, please use the conventional `if`, as described above. Even better, use the `.get()` method, like @gasull does [here](https://github.com/Ignat99/OpenBazaar/pull/1/files#diff-06caaf3cbf31ee2085d6189a8fe7e166R732).