I occasionally have clients ask if I do web design, and I say no, send them to partners that do web design and move on to the next SEO/M job. I often have developers or web designers call asking if I’ll partner with them because they have some clients that have asked them about SEO/M and they don’t want to do it themselves. I agree, send them one of my marketing page promotional bits and give them a 35% markup on sales of my services to try sell SEO/M to their clients. But only very few actually come through with those jobs because they aren’t good at selling SEO/M and usually underestimate the cost to their clients. They don’t understand the value themselves so aren’t very good at convincing anyone else to spend money on it.
Long story short – you gotta specialize.