Your sole proprietor will file on schedule C, and his buys and sales are reported on form 4797 for all his securities activities that are not for his investment, and he will have to file a 3115. I believe it is an automatic change with a DCN number. Most traders will have both personal investments, and trader activities. As for an Scorp I believe the only accounting change is for the Scorps securities activities only, and all other activities will remain as is, but he still has to file 3115. That is only my understanding for the Scorp, as I have only done a sole proprietor