I have found
ebay is a good place to sell, but it takes time to build a reputation. Stamp buyers only trust people they have dealt with in the past. Do you have an
ebay account?
Hip Stamps and Stamps2Go are also place that I have considered, but never tried to sell. Maybe some else has experience there?
"(20130207) Not allowed by the Staff" has a new marketplace with no fees at this time. They might be worth trying if you want to save on fees.
Good luck!