Here's my opinion, please read it since it's why (as of right now) I'm not going to change the link:
First of all: the Obvious
Anyone who posts something like that is obviously an insecure, pathietic individual -the type of person who comes out of his house/cave/trailer/etc. sees people who don't look like him, and flips out.
I find it greatly amusing that someone who supports "white superiority" deals with such a distinctly Japanese system as the Neo Geo. Guess he's a hypocrit.
Opinion
As for letting him put it in his sig, I want to side with Samhain. While it links to offensive material, (1) the link itself is not offensive and (2) the material leads to is not dangerous to (a) the well being of the computer (virii, etc) or (b) the well being of the user (no risk of getting in trouble for looking at material).
(1) The link itself is not offensive
Looking at the signature: it contains nothing more than a link. The user could have done much more offensive acts, like placing either a offensive symbol (swastica) or inflammatory speech. As it stands it's completely harmless (the URL doesn't contain offensive words), so lets move onto the linked material.
(2) The material it leads to is not dangerous
This can be sub-divided into things that harm your computer and things that harm your status at school/work.
(A) Computer harm
While the link is to controversial material, there's nothing harmful to your actual computer on the site, like a virus.
(B) Harming status
Here's where it gets complicated. There is no linking to objective material that will cause the person who unsuspectingly clicks on it to potentially get in trouble (reprimanded, fired, etc) at work/school for what pops up. The easiest example of this is Pornography. The only possible way for someone to be allowed is if they post a substantial amount of warnings around the link as to warn anyone who would click it. The question is, does the material that this user linked to qualify in the same category as something obvious as pornography. I believe not. There was a time, 30-40+ years ago where the whole "superior race/resistance" could get you into a lot of trouble (because a substantial amount of people believe it). In present times, it's the opposite: The overwhelming majority of people thing "racial superiority" so absurd that anyone caught looking at their materials is not assumed to be a member or have any interest in joining the group. While employers may have a problem with a employee linking to such materials on their business-related work (since that would reflect on the company), we are not employing him and he clearly doesn't represent us.
The flip side to this argument is, as has been stated above: Why should we, as a private website, permit members to post such offensive materials?
Well, we already do allow people to post what some view as highly offensive material in their normal threads. There are more than enough people who find problems with the Stars & Bars and what they represent. While this site will not tolerate Nazi symbols in a person's signature, I can't see allowing the Stars & Bars and not allowing a simple link.
I think, while this individual has a clearly illogical worldview, he did not post an offensive-in-itself signature nor is the material he linked to dangerous. The best this link might warrant is a warning, but I hesisitate to force that into the signature.
I have also noted he's staying strictly in the buying/selling and is trying to steer clear of any of the barbs sent his way. Those actions show that he does no intend use his signature to start a flame war.
If any other mods want to give their thoughts, please do.