Comment by cootsnuck

All the time. The company is usually begrudgingly okay with it too because they'd rather a former employee sell their expertise back to them instead of a competitor.

I guess check your employment contracts and whatnot, but it's very common for people to leave a company and start consulting and/or freelancing in the expertise they gained at that company. And it's not too uncommon for former employers to be one of their first clients or get your first clients from relationships associated with your former employer.

All of this assumes you don't burn bridges and have the interpersonal skills, in-demand expertise, luck, timing, and interest in building out a small service-based business.