I think to do this you have to be comfortable with the freelance lifestyle. Especially if you're not well-established / reputable in the field yet. Be prepared for difficulty finding work, and I've found freelance websites to be kind of hit and miss. My boyfriend is a freelancer (in a different but also similarly geeky field, I guess). He actually fell into it because the company he was working for decided to shut up shop, and the rest was history. He already had a few years of experience working for companies before that happened, which helped in terms of contacts. Even so, it can be feast and famine. Admittedly, he's not big not networking and marketing himself (which sounds like something in which you excel, which will be helpful, though of course you still need the skills to back you up). But he still gets work, sometimes a lot more than he wants, sometimes nothing at all. Luckily he's a pretty chilled out guy, has savings, is willing to work crazy hours when he's under the pump but also doesn't stress when things are quiet.
I suppose you're young and have no dependents so it's not a bad time to take a risk. If you are fairly certain you can get a part-time job while you build up your skills, that sounds like it could be worth pursuing, and you can always go back to sales if it doesn't work out? Teaching English overseas could be fun, too. :)