OFFERING FREE GUEST POSTS good or bad idea ?

It all depends on who will place content with what backlinks and what niche. There is a lot of sites working like that but You need people who have good sites (boosting sh...t make your site sh...t). And I would all the time check Copyscape if they are unique.

1. You need unique and relevant content for SEO.
Not crappy content that has already been published across multiple sites.

2. You need to understand how Google Page Rank works.
You want backlinks from sites with a higher PR than yours.
If they are lower, it may damage your PR.

I think these are the two most important points that you should think about concerning search engine results.

Hope this helps

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