I don't usually do marketing or even SEO, but occasionally... I get inspired. So I bid on a job for ten to twenty Google AdWords. The company produces a high-end sound system that has a lower cost than most. You can check it out here: https://www.facebook.com/pages/Nippon-Hifi/516180078478792 or here: www.nipponhifi.com to get an idea of what they do. Something about the way the project was written just inspired me. I didn't think I would get the job because I don't have any adwords samples, but I made one up specifically for their company and put it in my bid.
If you are good at slogans and sales media, that is definitely the way to promote your freelancing skills. Marketing is another great way to make money freelancing because so much of it is needed these days.
Google Adwords are a little tricky though. Usually Google Adwords are 3 lines and a website address. The first line is usually your keywords and your product name and is 25 characters, i.e. "SoulNote Sound System," or "SoulNote Hi-fi Equipment." Then there are usually two lines of 35 characters each - your slogan and your call to buy something (Try this/ get this/ check us out/ etc.). This is the formula you have to stick with, so it can be quite challenging.
In order to do this job, I had to not only look at the AdWords website, but I also looked at a blog that dissected good and bad Adwords. If you don't use your limited Adwords well, you can end up paying more for "bad clicks." So writing Adwords required a little bit of research, but once you learn how to do it, it is a good skill to market.