Taking someone to court for stolen property and money owed. She wants an itemized list, should I provide it or not?

This is your chance to show the judge what she took. This is how you tell the court to make her return your property or pay you money so you can replace your property. See RCW 7.60, replevin.

You are the party that is suing for what sounds like conversion of property. You have the burden to prove your losses which means not only a list of the property but also proof that this person took it and the value of the lost property. You need to give the judge sufficient evidence to rule in your favor and your opponent needs to at least know what items your are claiming as well as your other evidence. If she has different evidence, you can certainly demand it from her and should formally request it.