First of all...ask for a written marketing plan that outlines specifically all they will do to market the property. Interview several Realtors, but only let in those with a written marketing plan.
If you don't like someone, don't hire them...you will have to work closely with them and will need a relationship that is built on trust.
Compare the marketing analyses they provide for you (make sure they include written copies of the recently sold comps).
See how knowledgeable they seem...do they explain the market to you? Do they explain about mortgage contingency clauses and how they will affect the appraisal and ultimately the mortgage commitment of the buyer?
You can also ask Realtors outside of Manhattan if they have knowledge of anyone they would be willing to refer...referrals put a big onus on the listing agent to perform at top notch...or expect not to get any future referrals.