I'm not a lawyer but:
Privately-held companies have shares, they just aren't publically traded.
Estimates on the share value are made by accountants for private companies all the time.  If there is a difference between the option price and current estimated value then that difference is (roughly) the value of your options.
You can sometimes sell the options back to the company once they are vested, especially if you are leaving the company, but there is no law (that I know of) saying they have to.  
Option and share grants do often say the company has the first shot at buying them, but I assume they have to at least match any other credible offers.