Especially not an Explorer from a private party, with no boxes and papers. This is one of the best replicated Rolexes and with no date complication you honestly have nothing to go on other than a handful of minor tells.
They could have swapped the rep crystal with a gen or a Clark. It could have a gen crown, or even a gen bracelet. There could be any number Frankenized parts on there and even with some gen parts and a gen bracelet that seller still stands to pocket a grand profit if he sells it for $2K.
The big thing would be to see if the seller would take and post a video for you of him setting the time with it so you can see if the time advances forward when he screws the crown clockwise. Or even do a Skype or Google + chat and have him rotate the crown in the time setting position.
I think you are safer buying from a dealer on the Bay with lots of feedback for watch sales specifically. I would not buy a watch without papers from a private party but I would from a dealer. That's just me, though.