Well, in the cited episode Lost Boys, when Adalind asks Nick how long he has lived in the house he says:
About uh six and a half years. I never thought I'd be here forever, but I didn't know anything about being a Grimm when I bought it.
In The Good Shepherd Juliette offers to move out, which doesn't make a lot of sense if it is her home.
In Season of the Hexenbiest Nick gets fed up with Juliette and does move out, instead of continuing to sleep on the coach (even though they have a spare bedroom How do Juliette and Nick only have one bedroom?)
Let's say the writers might have screwed up and it is Juliette's house.
In Oregon, when it comes to unmarried couples and property.
- Each partner is entitled to their own property and debts unless they have been deliberately combined
- A written agreement to share assets overcomes the legal presumption of independent property
- Jointly owned property is owned in equal parts by each partner
It is possible that Nick and Juliette had a written agreement allowing them to share assets. This would enable Nick to be able to sell the home.
Also, even though they have lived together for presumably 6.5 or more years, there is no common law marriage in Oregon.