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You should look into using using Robots.txt to filter out specific querystring parameters You can use asterisks (*) to match the URL type above... Disallow: /index.php?a=reg

Yeah never set a folder's permissions to 777, as that would allow any script in that folder to be run and that is never good! Definitely avoid these instructions. You were right to consult the SEO Moz Q+A before doing anything.

Random presentation of items is not what you want.   You want to tightly control which items are displayed on your front page. Reasons to display items: Hot seller you want your customer (who might not have searched for it) to see. New Item you customer may not know about you want them to see. Items you want want a little extra juice to flow to so they rank higher in search results. Items you want to get crawled faster (often new items)