It seems clear that in principle eBay sellers are subject to either the consumer rights or sale of goods act respectively, depending on whether the seller is a company or individual.
But how does one claim against just the user name? I doubt that eBay would willingly be so cooperative, so is it purely a matter of filing a legal claim, perhaps performing pre action protocol or even serving claim form by eBay messages and then probably seeking an ex parts or default judgment if they don't engage and then ultimately asking the court to compel eBay to disclose the user's name and address?