I've never felt right about selling or giving away a book whose theology makes me cringe. Usually I would either keep it for reference or toss it in the trash.
This is what I did. The books that I purchased in my early days, such that houseparent and others here have listed; I threw the stuff in the garbage. I do not even want the junk on my shelves and it would of been more hassel than it is worth to sell them...and besides I would not want to be responsible for even being the one who sold the junk to someone and strengthend their false position anyway.
And this is not bad theology, but I had some stuff that was so technical - with Greek knowledge necessary - that I gave them away to a friend who went to seminary and had learned enough Greek to be able to use them.