Hey guys,
I've a app/service which is only for sole traders (UK) and I'm trying to target them on Facebook.
What is a sole trader?
It's someone who is self-employed and runs a business under their personal name.
Essentially like Bob Smith (operating as) Smithson Plumbers.
It involves no corporate entity/structure and all tax is done under your personal tax number, at personal income rates.
It's often used by tradespeople e.g. plumbers, electricians, etc. as you can wake up tomorrow and decide to run a business without any real setup, and even employ people if you register as an employer.
The obvious downside being you have full liability for everything and get taxed as personal income.
Thanks zeno for explanation. 
I have found one way that's have some promising results, but I need to collect more data and see if it's scalable.
I created a short (30 seconds) video and I'm bidding for video views (Video Views objective) with broad targeting (self-employed & self employment). Both interests together have more than 500k and then I have retargeting campaign for users who watched 75% of video or more.
Nice idea with the video...I'd also target specific groups of them - the plumbers, the electricians, and so on...