Asking serious question about a product that is for sale does not mean you are against the seller even websites have FAQ section.
Why do I need to buy this email address, what value or benefits can it bring?
This is for lazy, busy or what ever you calls it people that dont have time ro create emails, and also this is useful to people uses many emails to register for every each of their accounts but i think its very hard to sell
How much you sell each?
No, I don't think that is why he is selling them exactly. It's probably because of the names: bitcoins, cheapcoins, bitcoins4u. This is a bitcoin forum after all. But I can't imagine someone actually buying them.