I'd like to clarify the rule against hentai, because there are different contexts and meaning of the word. What the rule is referring to is "adults-only material", regardless of the genre, medium, or format. These are items with explicit, graphic sexual content or violence, and things that would get a rating of 18+, NC-17, AO, or have a label of "adults only". This is stronger than a label of 17+, or R-rated or M-rated content. Unfortunately if you list those items we will have to contact you and ask you to not include those in your listings. Sometimes this wasn't enforced in the past, but after discussion we must enforce this rule, keeping consistent with the way the swap meet is run at the convention.
If you link to an off-site page with your listings, or with additional listings, then the page in general must be suitable for viewing for all ages. If adult items are listed, then it is ok as long as the page solicits the transaction to complete outside of Kumoricon forums, and isn't just an isolated page that only makes sense if you viewed it from the Kumoricon forum thread. For example, if you have an anime store, it may have adults-only items in it, as long as the main site is suitable for all ages, but you would buy those items from the store, not by sending a PM on the Kumoricon forums. However, as part of the overall rule against listing adults-only content, you can't advertise a store or listings page that primarily specializes in, or lists, adults-only content.