I have a Rolex Presidential series, 18K gold, diamond bezel, with mother of
pearl dial.  The watch is about 2 months old and in excellent
condition.

What would be the most error-proof way (all steps) of reselling it? 
I'm wanting between $8000 and $9000 for it--just under what I paid.

I don't really want to sell it on ebay because high-end watches
usually don't fare well there.

You know where it was bought,  If so, you could ask that store if they
would take it back.  Just maybe, they could be willing to sell it on
commission.  That would not give you immediate cash, however.
 
Here is a site that sells pre-owned rolexes.  Obviously the place does
not buy them for what they are asking.
http://www.jewelry-wholesalers.com/

There are also auction houses that specialize in clocks and watches.