I want to sell my software to Half.com marketplace sellers. I want to
find Half.com marketplace sellers email. I don't want to contact them
one by one from ebay contact form.

If I were a Half.com marketplace seller, I would expect that the site
would give me protection from spammers. I doubt that a site like this
would last very long if it did not respect the privacy of its sellers.

You do have the option of joining the eBay Developer Program and using
that as your entry point into selling to Half.com members through
eBay. You also have the option of offering your product for sale on
eBay and/or Half.com.

Just remember that one of the guiding principals of Half.com is that
buyer and seller are shielded from each other to ensure a safe trading
experience and to prevent off site sales. You will find that the
information you seek is a closely guarded secret and that because of
the Privacy Policy you will not obtain this information.

I hope this helps.

By the way, as a Half.com seller I am curious as to what your product is.