It sounds like you want something awesome, not some random seo position.
Well for this, I would look for some excellent technical skills. In an interview or from your portfolio (i.e you blog) to show a high degree of discussion on how seo has changed, what you think for the future, and demonstrate a clean white hat top quality link profile for your example sites. Be able to talk about why you have done things the way you have.
I would also look for someone who has that buzz about them. I.e. you have some top quality technical articles on your own blog, you are active on SEO sites like Moz, and have a strong social following.
Now you say you only want part time work,... so the requirements may not be quite as harsh as I mentioned. Essentially, I wouldn't want a CV saying what you can do, I want real examples of what you have done \ are doing. Run a successful blog? Excellent!. I would also want to know about your experience in the employed SEO world (diy affiliate sites \ self employed client sites are different to employed work in a sense that you have more restrictions \ accountability \ procedural rules to follow).
Once you have all that prepared, and a CV tailored to the firm you are applying for (do your research i.e. if the firm does alot of local business seo, focus on local seo experience (Google places, and all the other stuff). Practice your interview techniques, including problem solving scenerios that might be put to you. How can you benefit the firm? SEO is always changing... you need to show you are up to date and relevant.
Now before you worry, you seem to have the basis to show all of this with your experience. You are able to showcase particular expertise in profitable seo areas (ecommerce), you are up to date by being active on Moz etc. Speak to your strengths, and have projects and the knowledge to back it all up.