The United Federation of Doll Collectors is a font of information about dolls so you may want to check out their site to learn of doll conventions and dealers. You could sell them yourself at an online auction site. If you have a local dealer who specializes in dolls or toys you may want to talk to them. A dealer will generally want 1/4 to 1/3 of the selling price.
Before you attempt to sell your Barbies, learn their values. Good Housekeeping and Marl & B have an online list of the most valuable Barbies. A handy book with identification and price guide that is up to date is: "The Complete & Unauthorized Guide to Vintage Barbie Dolls: With Barbie, Ken, Francie, and Skipper Fashions and the Whole Family " by Hillary James Schilkitus.