I would suggest starting with something you are very passionate and knowledgeable about. When you sell something online, expect people to ask you all sorts of questions. Sure, its easy to research the answers online, but what would you do if an expert on the subject refutes your answers and gives a totally different answer to the original question?
Prospective buyers might have second thoughts to purchase from you because they might think that you won't be able to provide the customer and technical support they are expecting.
So start with something you know and grow from there :-). Good luck!