We all hate the love letters, but the simple fact is that the unlicensed hobby as a whole is ILLEGAL and infringes on intellectual property rights...
Intent, contributory actions and the way the items are marketed can all lead to grounds for legal infringement...
Sorry to be blunt but the little "it was a free 'generic' item" game is just that a game... Basically if the average citizen on the street was shown your auction would he say "Cool a free generic superhero muscle suit, is included" or would he say "Cool a free (character pictured in the item being sold) superhero outfit is included" If it was the second then it was infringement plain and simple...
Also I have to agree that listing or talking about the 'other' properties, studios or sites is in bad form... IMO it shows your intent to infringe even more so...
Again yeah it sucks, but the WB is a protective studio always has been...
Get your Ebay account back or a new Ebay account, and list your suits for what they are generic muscle suits, don't put any pictures, text graphics, logos, etc. to owned properties or insinuate in any other fashion that they are anything more then a generic superhero muscle suit... And if your suits are patterned off a specific suit that is owned property then I suggest you create a more generic one based off the human muscle shape and form...
Yeah they might not sell as well but it will remove you from the bulls eye and give you a much better stance legally...
Again yeah it sucks, but the WB has taken the first move if you keep pushing the button, they could very well unload putting you in the poor house for the rest of your life...