Depending on the setup, companies can often block/disallow secondary market sales. It almost feels standard at this point.

I don't know if it has a specific name, but I believe it is possible to work around the company in cases like this. You basically setup a contract where you accept money and then agree the transfer shares as soon as they become liquid. No transfer of ownership happens beforehand so the company is left out of the process entirely.

I have never tried it, but I always thought it was an interesting concept since I first heard about it.