Entropy's advise would normally be very sound, except unfortunately you most likely will not find a shop willing to purchase those from you. Having worked for a comic book retailer, I can tell you that the store's official policy was not to touch that stuff with a ten foot pole as 1) we know what you were doing while reading it, 2) there's almost no market thanks to the internet, and 3) we didn't market that stuff because it would of been a pain to make sure kids weren't looking at / buying it. In my experience, this is generally the policy of a lot of stores. If they do sell hentai, then it's not going to be used.
Selling used masturbation material is never easy. You might try your local Craigslist and advertise it as "Adult Japanese Comics" so people know what they're getting into... Just be careful that you do not sell to a minor!!! Beyond that, you could sell them on eBay or use an app like LetGo. If it were me, I'd probably just throw them out. I know your friend probably wants to get some of his money back, but the market's just not really going to be there.