Whoever needs it the most. I am in the retail market so I understand what you are talking about. We do have people try to take our customers by underselling us all the time. It's called competition. There are two ways to handle competition. You can scream like a little baby and complain to your customers that they are not being fair because it cuts into your profit, and stand the chance of loosing more business. Or you can try something different to support your customers and bring them back to your place of business.
Maybe you guys should attack sites like ebay, who are probably doing some real damage, and leave small time guys like Brian alone. If you were smart you would leave him alone and hope that he sells out fast. Making him hang onto to it longer only drives the price down, but he will still have to move the product. So now he has still "take your sales" but nobody has made any money.