When buying an expensive, quality, high-end watch from someone you have never met (and are never likely too, for that matter) it can be quite nerve racking as there are fakes out there covering most off the well known makers but there are a few questions that should be asked that can help reduce the likelyhood of paying good money for a pile of fake junk.
1) Ask the seller to send a photo of the watch on todays newspapers and with a certain time showing on the dial i.e. the watch placed on 'The Sun' and reading 8.20 am/pm? (IMO it is only polite to ask the seller to do this at the start of the auction as it can take a little time to send this to the potential buyer)
2) Ask all the relevant questions and take nothing for granted:
does it function 100% correctly?
what size wrist will it fit up to?
are the original box/papers included?
what is the condition of the watch?
has it ever been serviced and is there papers for this?
how long have you owned it?
what is the diameter of the watch not including crown?
do you know the WR of the watch?
does it have a screw down crown? etc, etc ,etc....
The best advice I can give is use your common sense - if the pictures are blurred it can be a red flag (it's not that hard to post some decent shots with all the technology we have around nowadays) , if it reads something like - 'I found this watch while clearing up in a nightclub' OR 'I came across this watch in my grandfathers closet' then 99.9% of the time it is too good to be true. Remember not to be caught up in the auction fever and think that you have to have this watch - there are lots of good watches/sellers out there if your prepared too look and do your homework first.
Good luck and happy hunting!