I lived with my girlfriend, she asked me to leave and now I can't get my things. I fear she has sold some of them

Well, here is the thing.  You two lived together, so in terms of theft -- it really becomes a gray line. Do you have receipts to show these were only yours and not to be shared? It will be quite difficult to prove theft per se.  Why did she ask you to leave? More importantly, do you rent or own where you live? If you own, who is on the title to the house and the mortgage? If you lease, who is on the lease?

How much are the things that you indicate she took and sold? Do you have proof she or someone on her behalf sold them on the internet? On craigslist? If so, contact Craigslist and then contact the police.