Some people here seem to misunderstand that there is a legitimate business model involving guarantees with SEO services. When people say "guarantee," they really mean "money back guarantee" if the client isn't happy with the results. This is entirely viable if you increase your base cost to cover the refunds that may be required. If you have a serious private blog network, for example, it can cost you almost nothing to add a client's links to the site and wait for the results.If the client is happy, you made $1,000. If the client isn't happy, remove the links and you lost maybe $50 for the content and 1 hour of effort, but keep the content anyways, and many times the client will come back begging for the links back because their rankings dropped again.
This is just an example. We are a blackhat SEO/Internet Marketing website. There is nothing wrong with guarantees. We know how to move websites higher in the search engine rankings using blackhat methods. We don't need to be able to achieve a client's goal 100% of the time to be profitable and legitimate.