Hope I'm not breaking any rules with this post.
Since this thread is for newbies, and I am new to this hobby, can I ask the Sifus here this.
There are many shops that sell pre-owned watches. In your experience, how do you check or find out if;
a) the shop / seller is honest with his claims.
b) the authencity of the watch that you are interested in?
Thanks in advance for the advices 
Me no sifus, but from what I can gather these past few weeks,
a) the shop / seller is honest with his claims - "buy" the seller first, ie get "pally" with the seller and from there assess if he is an honest seller; find out from other buyers (if you know them), recommendation from internet(?).
b) the authencity of the watch that you are interested in -
1) do your own homework; for example, if I am looking for a 1970 Rollie, check out the serial no. and see it match the year of make or not.
2) caress or molest the watch, (this one may need many moons of experience by feeling the fakes with the originals) you will know if the watch is a "chou fo" (Cantonese meaning rough stuff aka fakes)
3) get watch sifus from MWN to tag along for their assessment
Some of the recommendation can be forund in the thread started by siodee on vintage watch and please see my comments at reply #13:
http://www.malaysiawatchforum.com/index.php/topic,2798.0.htmlCheers.
Chng