How well do you know Rolexes?
Can you tell if it has after-market parts, how old it is, and if the watch is authentic?
Rolex will not honor a non-AD warrentee (so be prepaired for major repair bills), and they will not work on a watch that is a "franken-watch" (one with after-market parts)
Also, there are a LOT of fake Rolexes out there. The general rule of thumb is: "You don't buy the watch, you buy the dealer."
In short, buying Rolex from a non-Authorized Dealer is a gamble. (JomasShops and their ilk are not ADs!)
How dangerously do you like to live?