What's your budget?
How many sites?
Competition for the keywords, roughly?
Any particular SEO method you want them to focus on?
Outsourcing is simply trading time for money. So if it makes sense, do it!
Elance is a great site, another one I've used is Odesk.
yes it is great one to do...but so many companies do it...so won't it be a problem at some point...like for eg. web traffic???