Sometimes you have great ideas but no talent for writing. This is when it becomes a good idea to hire a writer. Or you may be a very busy professional or expert in your field that is already really busy but needs an e-book or scores of SEO articles and have no time to write them. If either of these situations describes you, then you might consider hiring a ghostwriter.
There are several sites on the web where writers advertise their talents but the best two for ghostwriting SEO, blog writing and e-books are www.guru.com and www.elance.com. Both of these sites require a paid membership but it is well worth it as that fee (which is paid by both clients and providers) is there to keep the rank amateurs out of your face. The competence level on both of these sites is pretty high. On both sites writers bid competitively to get your job and you can assess a writer by looking at their comments and feedback. However keep in mind that both these sites are slanted to get the providers to do writing for as cheap as possible and many inethical marketers give bad feedback in hopes of getting a discount on their product. Talking directly to a writer on guru or elance and asking for samples is the best indication of whether you are hiring the best ghostwriter for you or not.
Also it seems that of all the brokerage or agency type sites on the web most freelance writers who specialize in web writing seem to have conglomerated on either of those two sites, so if you are serious about hiring they are definitely worth checking out if you are looking for high quality writing. Guru and elance are also the cheapest places to find SEO ghostwriters on the Internet as well.