Well people are obviously finding you and reading your useful posts, so they think it would be a profitable venture. Personally I would accept for your type of website as it would fit in great.

However Search engines are meant to be penalizing people for selling too many inbound links and becoming a link farm.

I'm not sure about price range, I wouldn't charge him much however he came to you, instead of the other way round which I would say is quite uncommon. My natural instinct is telling me about $10 every six months or something.

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