I have an old Intel® Core™ 2 Duo E6400 in a perfectly good system sitting in a closet at home and I can't even give it away. Unless you find someone who is collecting them you are most likely going to have a hard time getting any value out of them. You could try Ebay but again I think you are going to have a hard time getting anything out of them.
I think i'll probably just keep the i386 for later on down the road when it really is considered an "Antique".
I find myself extremely lucky to even see and own all of these since i'm still young.
Also, your Core 2 Duo E6400 matches the speed of my mobile Celeron
Socket 775 is still really capable even for being a dead socket.
I wanna work for Intel someday...