CAC is pretty much universally acknowledged in the field as a good second opinion as to the coin grade. If they feel a coin is undergraded, they will even put a gold sticker on it. There are several other "stickers", but the only one that I am familiar with that is considered of value is the "Eagle Eye" sticker that expert Rick Snow puts on Flying Eagle and
Indian Head cents that he considers are correctly graded and have good eye appeal. I think he also will offer a buy price for such coins.