eBay
EBay is the old stand by for people cleaning out their closets. There is also a healthy market for vintage Barbie clothes and accessories. You won't have to do much to get noticed here. There's already a built in customer base that can't wait to see what's been posted today. Make sure to look around and see what keywords are being used to get your vintage Barbie clothes noticed. EBay is the perfect marketplace for your more rare clothing items, as the auction format will only push the price up.
TIAS
TIAS is a site specifically for collectors and sellers of vintage merchandise to meet one another. The site is not set up for auction, but it is focused on vintage material. You will have to find out what your items are worth before posting them. There is also a registration fee. However, if you are looking for an extra good home for your vintage Barbie clothes, TIAS may be for you.
Vintage Shops
If you live near a major city or college town, there are probably lots of vintage shops near you. Go through the Yellow Pages or Yelp.com and see what there is around. Make the rounds of the different vintage shops in your area. See how much they will give you for your entire collection of vintage Barbie clothes. Make sure to compare offers before selling. This is the most inexpensive of all options.
Flea Markets
Go to your local flea market and talk to them about becoming a vendor. This is probably the most expensive option. However, if you are a looking at becoming a dealer, rather than just selling off your personal collection, this will be a great option for you. While lots of people use eBay in lieu of going to the flea market, some people love to browse and handle merchandise before buying it. Reach these people at the local flea market.
Garage Sale
When all else fails, there's the option of setting up shop on your front porch, yard, or driveway. There is little overhead involved, except maybe an ad in the local newspaper and some photocopied fliers. Tag your vintage Barbie clothes, set them up, and sell them to whoever walks by. While overhead is low, the downfall is that people expect yard sale prices to be bargain basement.