I think that it say invalid because it is not a common item that people use. I know because I tried to sell as well and it wouldn't let me put up the sign.
You can either use 263:1 like Sir_Creeper suggested or type Coal:1. I had this issue in the past. In order to get the ID of any item, hold that item in your hand and type /iteminfo. The server will spit out the name and ID number for any item.