S.T.
I used to know someone who bought/sold maternity clothes through the OC swapmeet. It's been years, though, so I'm not sure she's still there.
If you can't find a way to sell/consign your materity clothes, then I recommend you consider donating them to a mother's shelter in Long Beach called New Life Beginnings. They're on 6th street between Atlantic and Alamitos. Their phone number is ###-###-####. They support mother's of all ages and their children (born and unborn) by providing shelter, food, education, and so much more. Often, NLB is able to use the clothing. If they're not able to use it, they have a thrift shop that they'll sell it in that supports their mission. Either way, your clothing would support mothers and children who are down on their luck.