Hey, now that's a good idea!
Never thought of that as a way of getting links before.
Wonder how much you could sell that for? If you went on 10 forums with a good pagerank and offered to sell 4 websites links in your signature posts I wonder how much you could earn?
The websites would certinly get alot of high quality links from the pages you'd post on. And as long as you didn't overdo it I doubt the moderators would care.