First and foremost, lend credence to the saying: If it appears too good to be true, it probably is. My friend Nanuq has succintly summed it up, but there are a few points: a. Doesn't he have papers for the watch? b. Spend some time to have the movement checked. If its a Rolex 3135 movement, the watch is more credible... if the movement serial matches the case one. c. How well do you know and trust the seller? He is a co-worker, so you would likely be seeing him every day, or very often. Do you think he would scam you, knowing the situation that he and you are jointly in? Is he planning to move away or resign soon? d. Lastly, do you have any pics of the watch that you could share here? The Rolex experts would take it apart in a heartbeat. Caveat Emptor my friend!