Complex sentence: After the selling force of the boiler room sell their clients on the idea of the IPO, they are not allowed to sell the shares that the customer invested in because there is no real "market" for the shares.
Split into simpler sentences: After the selling force of the boiler room sells their clients on the idea of the IPO, they are not allowed to sell the shares that the customer invested. This is because there is no real "market" for the shares.