I saw a powerseller in Ebay that want to sell big silver coin with 2 black stain in size of 5 mm diameter, and he grade the coin BU. I ask if this is possible? Thank you.
Since BU means Brilliant Uncirculated and the coin in question has some staining or toning, then it isn't technically "Brilliant" Uncirculated. It can still be described as uncirculated with modifiers indicating the quality of the coin. Thus it might be called UNC, GEM UNC, MS-63, MS-67 or have some other descriptive wording. The answer to your question is no.
