SEO has evolved quite a lot. For example, in the past it was quite easy to get high rankings by using deceptive practices like keyword stuffing and spammy backlinks. You could earn a lot of money by ranking website on very hard terms like payday loans with nothing but spam and keyword stuffing. Unfortunately, these days are long gone. We live in a complete different time, where spam and deceptive on-page techniques don’t work as well as before.
Today you have to apply smart and clean on-page techniques. For example, Google nowadays is so smart that it can recognize synonyms. If you want to optimize your pages the right way, you need to add your keyword along with LSI keywords and synonyms, this is a huge signal which will reward you with better rankings on Google, and therefore more traffic.
Also care a lot about your loading speed. Take into account that images should have a relevant ALT tag, so that Google can recognize what they are about. These are the basics of on-page SEO, however there are more advanced techniques but discussing them here would take a lot of time.
Off-page SEO is a bit more complex nowadays. You have to care a lot about the quality of your links. The best way to get clean and powerful backlinks is by using the power of guest posting. Guest posting can give you access to great websites with a lot of authority and link juice, apart from the huge exposure to a big audience and potential buyers.
As you can see SEO has evolved a lot, things are not as they used to be back in the day. You have to adapt to this environment, otherwise you will die. Start doing SEO the modern way and you will be rewarded with traffic and sales.