The result of this small cottage industry is that employers will be tightening up their shareholder agreements and their stock transfer restriction clauses.
Or, they will react to the high demand from employees that wish to be able to liquidate their equity. This could cause companies to get more creative/competitive with their compensation. One of the great advantages to working for a company that's freely traded is that you can sell your equity as soon as it vests.
In some cases maybe, but hopefully most founders who obtain some personal liquidity in later rounds are not sadistic/hypocritical enough to deny their employees the same opportunity.
But you're right, one potential large risk is a Chris Sacca -like situation, where one investor/investment group uses many anonymous buying agents to acquire a huge stake in a takeout/IPO candidate, via secondary liquidity. That can mess up a final outcome for whoever thought they had control over the cap table.
Based on a comment further up, my understanding is people buying on the secondary markets are not actually buying the shares. They're just offering $X to an employee now in return for being entitled to the full sale price of that employee's shares ($Y) immediately after IPO. $Y could be higher or lower than $X (that's the agents risk) but at no time does the agent actually own the shares.
People buying on secondary markets often directly buy shares. They only resort to derivatives when they're unable to buy directly due to stock restrictions.
Yes, but the type of restriction matters. Sometimes it's just a ROFR at the same price, and that alone isn't enough to deter either buyers or sellers from directly buying and selling.