Nowadays, people want both web design and web promotion, not just web design. But if you're not exactly an SEO expert, should just try selling web design service? Is it a good way to approach? Or you should try learning a few things about SEO? Or you should work with an SEO expert / consultant? How should you approach if your strength is web design?
While there is certainly need for dedicated designers, my opinion is that SEO can not be separated from design and that many sites are beautifully designed (especially Flash) by designers who don't "get" SEO. These beautiful sites may be invisible to the SE's and therefore visitors My advice is to study the subject, so you can offer the services. SEO is relatively simple, and from a design standpoint, all you really need to know can be learned by reading a few e-books. If you don't want to offer web promotion, don't, but designers who are ignorant of SEO really can make it hard to do SEO later. Offer value to your clients, get to understand on-site SEO at least and leave link-building to specialists if you want to focus on design. But you need to understand navigation structure and especially crawlability. SEO for Dummies is actually a great primer. Others will recommend their favorites.
Learn as much SEO as you could. People make websites to have their products and services found by as many people as possible. Customers hope to recieve high amounts of traffic and if you could help with that your web design services will always be in demand.