Although I haven't used this supplier personally, they seem quite legit and well established. They have been online since 2003 and accepts PayPal payments. I was also unable to find any negative reports about them.
There is no mention of any documents to prove the authenticity of their brand name watches, other than their reassurance that these items are indeed for real. So I would highly suggest that you try and contact them first regarding this prior to making any purchases or even try and visit them if you can. Here's their contact details:
Write to us:
1335 South Hill St
Los Angeles, Ca 90015
Hope this helps. Cheers! :)