Answer by Ian Christian Myers:
What follows is not Zappos official policy or anything like that. Since we're treading into the territory of legality and Terms of Service, I feel like I need to state that I don't represent Zappos in any official capacity and certainly don't speak on behalf of the company. Each employee goes through customer service training, regardless of position. My answer is based on how Zappos generally operates in a customer service capacity and on the training I received.
That said, the only reason I think Zappos would ever blacklist a customer for amount of returns is if we could prove, beyond any doubt, that a user was using the site in a malicious manner (i.e. they are trying to increase the operating costs to high level by using the shipping and return policy against Zappos), or the customer is continuously returning worn items. In other words, you'd have to violate the terms of service in a very obvious way, repeatedly, with multiple interactions with our customer service team.
For worn items, it would have to be glaringly obvious that the item was worn and then returned (e.g. stains, torn, smells, etc.) and it would have to happen several times. The first few times we would most likely assume the customer received the item that way and that's why they were displeased and wanted to return the item in the first place.
In terms of pure volume and percentages, at an individual level this would be very hard to do. The Zappos site, back-end systems, CRM and CMS tools, and warehouse are designed to scale to a very large volume. A casual customer, ordering a normal amount of items, and returning most–or even all–of them, most likely would not be noticed amongst the thousands of items we process every day.
As you start ordering more and more, you might start hitting some of our safeguards. There's a cap on the amount of items you can add to your cart and check out with. It's a relatively high cap, so you might still rack up a pretty high dollar amount. If that dollar amount gets high enough, we'll probably call you up and have you verify your order. If you were to do this a few times and return everything each time, we might start asking you what was wrong with what you returned and how we can help you find something you'll be happy with.
If you try to get really malicious and distribute this out, you're bound to get caught up in our fraud rules, which will also get a phone verification of the order.