Do know or what do think about a user valuation hack like ebay?
Users can give a valuation to other users posting and this user add this "points" ti his profil. And new users can see, what value this user have. I think, the number of postings and the quantity (number of chars of a post) can flow in in this valuation.

What are yout opinions?

This is a good idea, but a bit difficult to implement fairly and securely, so abusers won't take advantage of it. Slashdot has their method. It's a bit convoluted, but it seems to work.

Is there no existing version for this problem?

Idea A:
New user start with a point account of 100 shells (better as points?).
When he post, he get for every word a shell (or with an multiplier). For every participation at a poll the same (with an other multiplier).
When he get a good validation from a other user he get shells and when he make a validation for a other user he lost shells.

Idea B: (simple)
A new button under every post. A user can bit this and some shells where added to the profil of poster.