If I do have a website which sell products like Phones or tablets, I'll definitely targeting long tail keywords which do convert much better than the general keywords, but when you are thinking about increasing the sales volume, you have to look into other factors as well, Does your site design appealing, Does your website has Call To Action (CTA) buttons, Do you understand the mentality of your website users. I could write a book about how to increase sales volume Hope that helps a bit!