Well, in all honesty, used bikes here to me seem to be very expensive. Especially the ones made in Europe. Japanese bikes are a bit cheaper. But compared to the second hand market back home (Greece), prices are high.
Having said that, if you're not planning to stay for long, I would advise you to buy a bike here, and sell it. You may only lose a couple of thousand dirhams.
Importing your own bike is another option. When I was looking for a bike, I spent some time on google, and found out that there are a lot of shipping companies that can import your bike from the states and do all the paperwork for you (if you decide to follow this route, then definitely have them do that) The import tax here is 5% of the market value of your motorcycle. So , if you add up the cost of shipping to UAE, import taxes, cost to ship back to the States, and re-importing it there... I think you'd be better off buying a bike here.
Third option would be to import your own bike and then sell it here before you go. That would cut your expenses down significantly.
I guess it all depends on how long you're planning to stay here... Oh, and expect to pay A LOT MORE than $8000US for a second hand F8GS...
XTZ 750 '96