Children from the ages 7-puberty are such interesting creatures. Their need for love and compassion is actually heightened at this stage and their desire to have control over their person and belongings is very natural.
Every child is different. For some, disposing, or handing down of no longer needed items comes easy for them. Where as with some, arbitrary disposal of their beloved items can actually be quite traumatic as these items may offer a sense of security. If the items are being used as a control mechanism, then you will need to find a way to work through the underlying issue.
Since your daughter is willing to sell her items, then let her and use this as an opportunity to teach and learn. Have her set up a mock store or garage sale and give her stickers to price accordingly. You can take her to garage sales in the area so she can get a good idea on pricing. Set a date. Have/help her make signs to place around the house. Make sure that she has a box or envelope in order to make change & a paper or receipt book so that she can add up all of the items that she sells to you. This will help with learning money & business.
As she is beginning this process of sorting & labeling, mention that you are available to assist if she needs anything. She may take you up on the offer & this will help with your bonding. If she doesn't, then she may be trying to prove that she is a big girl in control or even that she is doing something for you to be proud of, all by herself. Either way, as long as she knows that you are available, willing and supportive of her decision, you are keeping the lines open & building your relationship with her.
After the process, ask her what she would like to do with the remaining items. Oh, don't forget to praise what a good job she did. It was a lot of work after all. Offer one suggestion, such as "Did you know that there is a shelter where kids and their moms go when their dads aren't so nice to them? Sometime they have to sneak out in the middle of the night and they have to leave all the things that they love and they never get them back. I know that these clothes(toys, supplies)would really be loved if that was their new home. It is just a thought." Give her time to digest that information. She may not jump on it right away. She may just want to bag them up and keep them. Fine. Let her & happily offer your services in help put them away. Treat it like her decision was not a big deal. This offers support & love.
It is likely that she will come back to you and say that she may have a few items to give to the shelter. Tell her that's great and then mention that you may have some things as well. Take them together & don't forget to mention how proud you are of her decision. This may work the first time, it may not, but if it doesn't don't give up. Persistent love & support will begin to manifest in a more willing child.
Warmly,
S.