The main problem seems to be revenue. With 4chan, you could probably do a unique form of advertising -- companies pay ~$100/cpm to NOT have their products or brand featured on any 4chan pages. :)
I'd imagine that kind of adult product would perform pretty badly on 4chan. The audience is definitely not in buying mode when they're there, and it's not like most people are going there explicitly for porn, so they don't have the kind of "motivation" that drives that kind of purchase. Probably things like cool / novelty items would do better there, things that appeal to teenagers with not too much money.
I've not seen an ad on the Internet since 2006. Apparently I'm evil and I'm bringing down the economy. If that means fewer jobs in advertising I count that as a win.
Not to perpetuate any stereotypes but fleshlight / masculine enlargement products and so forth would be happy to advertise there.