Keyword research for e-commerce websites goes far beyond search volume and competition analysis. The reason for that is the complexity of the buying cycle — each step in it calls for different keywords you'd need to target.
Typically, customers of an online store go through 5 stages before making a purchase; different keywords and keyword phrases are used at each stage.
Compile 3 separate keyword lists
As you can see, keywords that customers use at the Research, Comparison, and Purchase stages offer the most value in terms of conversion potential. To get a comprehensive list of those, you'll need to come up with 3 groups of keywords that you'll later combine into long tail queries.
But before you start, remember to research search patterns typical of your target audience: consider gender, age, and social status. For example, if you are a male and you are struggling to get the organic traffic for a skin care store, take care to talk to your female colleagues or friends to find out the jargon they use when they talk about this stuff. Spend some time on relevant social media resources to learn your audience's language.
When you're positive you understand how your customers talk and which words they use, get down to putting up your keyword list.
Prepare a list of action keywords that customers might use at the Comparison and Purchase stages as part of their query. Don't add the product or category names to these keywords yet. E.g. "buy", "purchase", "price", "compare", "review". Get a full list of brands available at your store. E.g. "Sony", "Samsung", "Apple".
Compile a list of categories, product names, and their core properties, like size or color. E.g. "TV", "laptop", "smartphone"; "Iphone", "Galaxy Note", "34-inch display".
Mix your keywords up
Once you've got these three lists ready, it's time to move on to putting together search phrases. Combining generic keywords with product keywords and their properties should give you dozens of long tail keywords — like "buy 42-inch Sony TV". It works like a slot machine: you turn the reels and get new keyword phrases.
Voila — you've just saved yourself a couple of hours!
Keep in mind... You can do it manually if you need to mix up a dozen of keywords. However, given the size of the inventory in most online stores, you will likely need software tools to get things done quickly.