It's fairly common, in a strong series C round and beyond for founders to take money off the table, especially when the VC appetite demands it. Typically, founders can sell as much as 10-20% of their vested shares, which can be worth 10s of millions of dollars or more.

I can confirm this goes beyond the founding team, I've sold shares as a part of raising capital at the last two places I've been employed. I was an early hire at both and held the CTO title. Series C in 2014 and most recently series B at the start of 2018. I also seek out opportunities to unload my equity in the secondary market, but I'm usually taking a haircut there vs the premium investors that are looking for a bigger share will pay during a capital event.

I'm a bird in hand guy when it comes to equity at the fast-growing private companies I tend to be attracted to. I'm almost certain I'd feel differently if I had a larger stake or founder-level attachment to what was being built.

Out of curiosity, how did you find investors to sell to for secondaries?

Founders often get the opportunity to cash out some of their stake in the company in mid and late VC rounds.

Sometimes VCs will purchase founder equity for cash.