A tax/business ID is a legal requirement to conduct business. I recommend you read, Link hidden: Login to view
It offers detailed information on why you should get a tax id.
As for your second question: What to sell. This is a very common question of those who want to get into online selling but has no prior business experience. My advice; always start with what you know or what you love, i.e if you love shoes, selling shoes online is a breeze because you already understand what your product is. This part is important because customers can ask you all sorts of questions and they expect you to know the answer all the time. Customers feel more confident purchasing from a seller who knows their product inside out.