Link Popularity and Contextual Linking are the cornerstone of Off-Page SEO, and not even Google can change that without a revamp of the internet as we know it. On-Page SEO can only get you so far, primarily serving to establish relevance. But the real weight is in relevant linking, and always will be. That is why anyone caught purchasing links will get the Google Slap. Caught... So if these Link-Trading networks come up with a way to make the process anonymous up-front, I think they'd be in business. If the service could verify things like PR, number of outbound links and relevancy, leaving that info in an open marketplace while omitting domain name or anything traceable to the link publisher, they would provide the buyer everything he/she needs to know while protecting the seller. And Google wouldn't be able to jump on the network, see who's selling and slap them into oblivion...