Without building your code myself, I can't be sure, but...
It looks to me that this is a simple 1-to-1 relationship where your
EJB either has a related ItemTradeIn or it doesn't. Am I right?
If so, then it looks like there's an unnecessary target-multiple="yes" on @ejb.relation. I'm also thinking that the @ejb.value-object setup you have is unnecessary for a simple 1-to-1 relationship. Once you remove this stuff, it looks practically identical to some code that I have that I know works.
But again...without building and trying your code myself, I can't be certain what's up.