If you want them to go to a decent place where someone will respect them then Amazon is probably the place rather than ebay etc.

Some aviation dealers around so maybe there but you wont get much for them.

Ask nicely if you can list them on here or maybe some members will show an interest and PM you.

Depends on the titles really.

Play.com is another quite good one.

PM me with a list of titles and I will ask around. As I said its all down to the books.