Fix comments and unittest for market.py and style

Trying to make the right commit to  the develop branch.
This can be prevented with this refactoring:
```Python
seller = msg['Seller']
seller['seller_PGP'] = self.gpg.export_keys(
         self.settings['PGPPubkeyFingerprint'])
seller['seller_BTC_uncompressed_pubkey'] = self.settings['btc_pubkey']
...
```

I'll add this change to my PR https://github.com/Ignat99/OpenBazaar/pull/1