You need to be careful of the term 'Grey Market'.... as in the watch trade 'grey Market' is similar to car trading from main dealers..
Many watch dealers hold a franchise license to sell the genuine watches these are issued by the marques themselves and are subject to the usual sort of regulations such as presentation rules, positioning in windows etc etc but most importantly meeting sales targets... if these targets are not met then the marque will rescind the license to sell...
Therefore many of these ADs sell the gens out the back door to 'traders' we shall call them as an example.. thats a big reason you see many gens being sold with blank warranty cards... stamped by the dealer but not filled in...
That way the ADs hit their sales targets but dont make thier profit on the watch if sold off RRP hence these watches can be sold in large quantities by ADs... so a trader would get a call and be told he can buy 4x Rolex SSDs 3xPAMs, 2x Breitlings, etc etc but he must buy the lot... regardless of whether the models are popular in return the trader will get a substantial cost reduction..
As I said its a bit like the main car dealers sometimes the greater value to them is in the overall sales rather than the individual sales... That is referred to as the 'Grey market'... I know a few guys in this game and they make great money but they have to sit on a lot of thier investment for long periods... so cashflow can be pretty irregular ...