The truth is that the Risk API just provides a simple return value to the customer (Virwox) which the customer can disregard, ignore, override, etc.
Virwox willfully blocks people based on the API result, regardless of how old your account is on Virwox or how old your Second Life account is. They patently deceive people in their FAQ about this, and when confronted with this, they spin spin spin.
As if their horrible fees weren't bad enough, this is enough reason to avoid Virwox at all costs.
From Linden Labs (via email):
There is nothing wrong with your account. You are authorized to trade on the Linden Exchange provided by Second Life. However, the third-party exchange is permitted to decline a transaction with your account, especially if your account is shown to share risk factors with other accounts that have been problematic in the past. As you already have a zero balance, you might consider adding a valid form of payment to your SL account and making at least a small transaction on the LindeX to improve your risk status.
From Virwox Support (via email):
Of course, we are free to accept that risk (that's what [Linden Labs] say below), but we won't.