I was just about to make my own post when I found this, and I have a similar issue. I found that one of my sims inventory items couldn't be moved or sold from inventory. I could only click each inventory item separately, click "place in world" and then the game said (sim name) doesn't own this item. For example bugs collected on vacation, collectible dolls and so on. I didn't know why this happened, but reading the original post here it must have happened after my sims moved in together. In my case the sim that already lived on the current lot asked his girlfriend to move in with him. I guess there's no way to get the items back as "owned" by my sim? :/ PS. any collectibles or items I got in my sims inventory after the merge of households, I can succesfully move or sell.
edit: I was just thinking, when my sims just moved in with her partner I DID successfully put some of her inventory items in the house. They are still shown as owned by her. So the merging of households couldn't really be the issue, else it wouldn't have been possible at all. :/ not sure ...
