ecommerce / 301 / domain seo

I have a ecommerce site
And 1 of the categories on it does quite well but i have no organic traffic to the category (i am getting ppc visits to the category only)
Because its a good category i want to open a new mini store based just on the stuff in this category
On sites i have done b4 i have found its very important to have your main keyword in your domain to get ranked and on my site the keyword for this category is not in the domain .
I can buy a site from some one that ranks well for the keyword i want (this is a amazon aff site) but i don?t like the domain. (Its a .org.uk )
if i make a 301 to mysitedotcom/category would the category site start ranking as well for the keyword as the site i am buying
or
should i make a new mini site with a new domain with the keyworld in it and make the 301 from the site i but as well
or
keep the site i am buying as it is and instead of linking to the amazon products make the products link to my site
hope you can follow this

thanks

Personnaly, i think that the last option is the better one.

keep the site i am buying as it is and instead of linking to the amazon products make the products link to my site

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I would go with that option, if you change the site's content it will likely lose some rank.

Your best bet is to buy the website and 301 redirect every page to it's relevant page on your website. This should strengthen your domain along with your individual pages and you should see an increase in rankings/traffic..