The two main factors which should influence what kinds of eBooks you want to sell are (1) the target audience you want to sell to, and (2) your interests, passions, and knowledge. There's no point marketing at readers that don’t exist or trying to sell a genre of books that you know nothing about. That said, there are a number of genres and topics that are always popular among eBook readers. Non-fiction titles about diet and exercise, self-help, crafts and cooking, education and teaching, religion and spirituality, and politics are trending all the time. And as far as fiction goes, try young adult fiction, supernatural and fantasy, erotica, science fiction, and thriller eBooks.