Hi guys, thinking of buying a used rolex datejust, my price...

Hi guys, thinking of buying a used rolex datejust, my price range is about $2000-$4000, anyone who knows about watches have any advice about things to check on the watch before buying, or any questions to ask about the watch before buying.

this is any EG. of one id be interested in. thanks anyone.

http://www.ebay.com.au/itm/ROLEX-DATEJUST-MENS-SS-14K-YELLOW-GOLD-WATCH-16013-/330560039590?pt=Wristwatches&hash=item4cf6e9eaa6