Ideally, no payment would ever get withheld. The proxy would maintain its own support database and build workflows for the most common disputes. So something like this situation would no longer happen, because they'd have the solution on file from any previous customers who went through it. Kind of like Stack Overflow for disputes, except internal so that customers don't have to deal with it. Vendors could even get access to the database to have better internal controls and avoid the snafus that lead to bad press like this.
Otherwise I just worry that every insanely great web company will inevitably turn into the next monopoly and we'll never get free of resorting to HN and lawyers.