If it has any trace of wear, it is AU. BU coins come from unopened original rolls. You can call a coin anything you want, but you may get called on it someday. Not a good way to build up a reputation. I know alot of people on Ebay get away with it, I suspect when the customers go to sell the coins they find out and get disappointed. I can see at some point in the future that people will be shying away from Ebay as they learn more about coins the hard way.
Best bet to maintain a good reputation would be to only call those coins BU which you are absolutely certain are BU.