E.A.
I agree with the previous response. Craigslist is a great option. You can do a search on maternity clothes in the kids/baby section to get an idea of what others have posted. If you feel weird about people coming to your place to see the clothes, you can arrange to meet in a public place.
If you don't feel comfortable selling on CL, you can try to take them to one of the several kid/baby directed consignment stores around Austin (Rock-a-bye Baby, Kid-to-Kid, etc.), however, you will probably make more money if you sell your items on Craigslist.
Good luck!
E.