1. An idea? Risky.
2. + funding? Less risky.
3. + proof of concept? More less risky.
4. + an MVP? Less risky still.
5. + paying customers and metrics? Less risky again.If the risk is too high they are all happy to let you carry it for a while.
From stock market investors seem to happy to be paying a lot for companies that are unprofitable or turn out only small profit. For unlisted companies, if you are willing to scout around you will find profitable companies where employees are holding stocks that they don't really see much value in - or they would prefer to have cash in hand instead of the future dividends. For smart investors who are willing to do a little bit of snooping, leg work and negotiations, these situations can be sometimes quite fruitful.
This is a separate thing from the natural evolution of risk over startup life. That happens to every one. Viable secondary markets letting you take some money out early don't happen in most cases.
If the company is valuable and you want cash out fast, then you can just send messages directly to investors, if you have any contacts. There are always some investors looking for that kind of deals. However it has to be significant value, like 100k +. And naturally the price will suck, if IPO is in the horizin it makes almost always sense to wait.