how do you discover good retail categories?

I have a question about how you would suggest an e-commerce retail company identify new product categories to get into.

Bottom line is how do you discover good retail categories unrelated to what you are selling

So - I am wondering how you suggest a company research a new category first? Besides the obvious tools like google KW tool or google insight or other similar KW tools. If I am selling shoes, maybe I want to sell washing machines on another site - so I want to be totally open to any category. I mean how would I know in the first place that washing machines are a great category??? Thats the key

It there a tool or service that identifies the hot selling categories online for retailers?

In the past we have gone into hot categories like GPS and even Vacuum Cleaners - both with tons of search volume. But competition was super strong and margins really poor - even if you inventory product.

Any help here to have a standard way to find new categories would be great and I am sure your other readers would be interested as well. If the information is really good though feel free to just send it to me

thanks so much

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Luke