Liz,
Peaches76 had a great idea. Buy some blank videos and copy the movies for the child. We did that for our niece. We had a bunch of Disney movies as well as other kid's tapes (Barney, etc) that my mom gave us because we are planning on adopting. Besides the fact that I love Disney movies...LOL
Anyway, we did loan some of them to my niece and my sister-in-law has yet to bring them back. She's bad about selling EVERYTHING at garage sales and I fear that's what's happened but that's another topic. Anyway, we learned real quick to copy the tape for her to take home to watch but keep the originals in the house. Those aren't allowed to leave. It's cheaper and usually costs less than $1.00 a tape.
As for the books, gosh that's a topic close to my heart because I am an avid reader and cherish my books. I'd let her pick out 5 of her favorites and leave it at that.
Not to sound harsh but her father should be the one who will continue her library of books and tapes and whatever else. You are not responsible for providing a whole furnished room in her new home. Since her birthfather took the time and effort to gain her in his custody, then he should also take on the other side of the equation and that means opening up the wallet to provide her with what he feels is appropriate.
You do have to think of the future children.
The guinea pig...I would talk to the birth father or tell her she should discuss it with him when she moves in because some adults do not like pets of any kind including the small caged ones. LOL... She's young so she doesn't realize it but the adult has to be the one to make the decision on what is allowed in the house because more than likely they will be the one to end up caring for any pets or animals brought home.
The sheets...I would let those go. I know it's tough but they are a sense of security for her and one set of sheets should be okay to let go of. This way, when you get another child and they are old enough, they can help to pick out what they would like. If they are only foster children, go with solid colors instead of character sheets so they can be reused. Instead, if the child really wants characters or you want to get them a character, choose a pillow or throw blanket. It will be easier for them to carry it with them and won't hurt so much in the pocketbook when you send it along with them.
The other posts are right in saying she is young and doesn't have a real sense of how to express things verbally. So she goes with what she knows...material possessions. Kids equate stuff with how much they are loved until they learn there is more to love and self-esteem than just the things they have. She sounds pretty worldly also so don't underestimate her. She probably knows more about manipulation than she let's on even if she can't exactly verbalize it.
If you are seriously concerned about her behavior and thought processes right now, contact the social worker and talk to them about your concerns. Also so if there is any possibility the child could get in to see a counselor or therapist to help the child better navigate her course.
By the way, keep the R rated movies in your house.
That's it. Keep us posted on how things are going.
L.