I have had reasonably good luck buying and selling used sails through Bacon''s in Annapolis. Bacon''s tends to get a lot of reasonably new sails from race boats. These sails have often lost some of thier shape but are half way decent sails to use on a cruiser. I have bought sails to use for winter ''messing about'' which, with the greater density air, can be really destructive to your good sails.
On the flip side, I have bought brand new sails, with boat show pricing and prepayment discounts quite cheaply from name lofts and they have been surprisingly close to some of the prices that I have seen for nearly new used sails or no-name lofts.