Trading shares as a sole trader

Hi everyone. Is it more beneficial to trade as a sole trader with an ABN? When it comes time to submit your tax return what are the pros and cons for having an ABN? For example; I don’t understand what having “stock on hand” means, and how it is assessable income. Or if I hold 400000 shares in a company that I have not sold but the share price has increased; is that taxable? Can someone please educate me. Thanks.