grey market in securities terms usually refers to OTC MKTs Caveat Emptor (buyer beware) securities, which are usually the bankrupt or super sketchy companies. If they were trading on here then you would know because there would be at the very least a symbol you could look up a bid ask quote or trading history. The other thing a grey market could mean in this case is the physical trade of the stock certificates themselves. If your shares are direct registered you should be able to ask AST to make you a physical certificate of the shares, then it’s just a matter of finding a person willing to physically buy it from you and then signing the stocks over to them on the back of the certificate. Only problem is if your shares are currently tied up with a broker then your shares are not held with the transfer agent directly so this may not be an option.