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Hi Laura.

For the structure take a look on this guide. http://searchengineland.com/best-practices-in-e-commerce-seo-176921 At Question 8 you have good recomendatios for categories estructure.

About using subdomains - domains - folders, I prefer using folders, so you will have the same domain authority in all folders. Anyway you can read this excellent article about it: http://moz.com/blog/understanding-root-domains-subdomains-vs-subfolders-microsites

Good Luck!

Hello,

I'm totally agree with Ryan. Good Luck!

Hello,

I would add to Oliver's tips having a good SEO onpage

Abu,

Yo won't go to a sandbox unless you have duplicate content, poor quality content or automatic generated content. Don't worry about it.

Hello Abu.

I think is hard getting good rankings only with content. If you create good quality backlinks you will help Google to evaluate tour website.