There is a developed secondary market for venture-backed companies' stock. (Source: I do this.)

I thought private stock transactions were subject to board approval, or is that just Canada?

It depends on the company. Some companies are quite liberal with transfers. If you have a bona fide offer, they'll amend the cap table. For others, my having a relationship with the founder or Board is critical. A good starting point for American companies is is their Certificate of Incorporation. If you exercised options, the Restricted Stock Purchase Agreement should also contain terms, e.g. the company's transfer policy or right of first refusal.

I have never heard of a Canadian company looking to raise capital structuring itself under Canadian law. Commonly, you'd have a Canadian law subsidiary wholly owned by a Delaware corporation.

Disclaimer: I am not a lawyer. Hire a lawyer before selling private stock. Do not take investment or legal advice from my Internet comments; they are neither.

There's an EU startup that's trying to create a blockchain based market for shares of pre IPO startups: https://markets.funderbeam.com/market

Depends on the terms. In most startups board can only match the offer.

Lol.. good luck.

Not convenient, but definitely possible. Equidate is a fairly open market for that kind of thing.

That is an exit of sorts.