Cleanup ignat99 feat

New PR from https://github.com/Ignat99/OpenBazaar/pull/1 directly into OpenBazaar:develop.
Since `item_price` can be `None`, it 'd like the code to handle it explicitly. You see, it is true that `None < N` for any positive or negative number `N`. This would have you believe that `None` is minus infinity, but `float(None)` raises an exception. I find that aspect of Python disturbing. Please rewrite the line as:

```python
if item_price is None or item_price < 0:
    item_price = 0
```

This states the error conditions much more clearly, IMHO.