Dan,
I am a member of several O gauge grips on Facebook. Some are quite good for sharing information like we do here, and some are buy and sell. I have both bought and sold and have had good success. Of course you have to watch out for the occasional person who isn't trustworthy. It is a common thing there to post a comment about how good a transaction was with so and so. Also once in a while a guy gets called out for a bad deal, then others chime in. Once you are there a while you know who to deal with. I've been pleased.