This is a very common question, and the short answer is - work hard and don't listen to your naysayers nor your lizard brain.
Starting a business doesn't require any capital, but often times many entrepreneurs aren't skillful enough to couple swagger + brute determination + insight. You have to know that it isn't about what you have, what you don't have or who you know but about how willing you are to pivot your efforts, try the same thing 3 times 3 different ways before saying that strategy didn't work, how good are you at negotiating distractions and negativity like self doubt.
I have helped several companies launch out of pure ideas and coached entrepreneurs to hustle their way to obtaining an MVP or minimal business product inventory to generate sales, in some cases you can sell before you even having a product. It all depends on the person, how likable that person is and whether or not the entrepreneur can be laser focused.
When you don't have money then you need time and clarity (that's why communities like this one here are so vital)
When you don't have time or money, then you need swagger and dedication to be everywhere, meet as many strategic potential partners as possible, learn from them, build relationships and get liked - is not about you who know but who you know and if they like you.
My name is Humberto Valle, I'm the co-founder of a global digital marketing agency (www.Unthink.me) and I have been working with startups for about 10 years and launched and exited 2 successful (no big deal though) startups myself. Best of luck, if you have any specific questions follow up and send me a message.
Do you really mean with an investment of time instead of money?
There are lots of agency, affiliate and brokerage opportunities such as travel or booking agent, freight forwarding, equipment brokerage, etc where you can earn a commission or fee from helping someone else make a sale or purchase.
What types of things do you know about that others don't? This could be an opportunity for a purchasing agent business for example.
The possibilities are endless however it's difficult to scale this type of business in a meaningful way and you may just end up creating a job for yourself. If your goal is to get out of a job you don't like by building a side hustle which can lead to a full-time thing then it may be a viable strategy.
Give me a call if you'd like to explore in a brainstorming session.