Or, who knows about a large sum of money hidden in the walls (or the like) — much larger than your house is worth — and they’ve heard you say on multiple occasions that you would definitely move into your parents’ home after they die?
Or more likely a variation on this: Since this person (“neighbor”) doesn’t know they have this “motive,” they must know that there’s something valuable about your house/property, and they’re hoping to purchase it from you, but they don’t know you’re planning to sell it when your parents die (they’ll just be happily surprised when it does happen).
I’ll pose this a question: “Does this person know about some value to your house or property that you are unaware of?”
It could be, say, an untapped petroleum energy source underground.
Do you own your current home?
Is there some kind of rent control on your property that would allow the landlord to raise the rent substantially if you moved?
Is there someone who wants to do something with your property, but you currently prevent it?
YES. This is the solution.
I had no idea that this puzzle would be so difficult or frustrating. My landlord is very unhappy, because the discrepancy between my rent and the market rate for my apartment has cost him hundreds of thousands of dollars, and will continue to cost him as long as I live here. I was thinking the other day that at this point I am unlikely to move out until I receive an inheritance from my parents. It then occurred to me that although he doesn’t know it (he has never met my parents, and as far as I know does not know anything about their financial situation, or that I would probably move out after their deaths), he has a very good motive to kill them. At first I was thinking it would make a good mystery story or episode of a procedural, but then I decided it would be a good lateral thinking puzzle. I assumed that players would be able to quickly determine that I would receive an inheritance, and then the hard part would be figuring out what I would do with the money.
One final note: Obviously the easiest thing for him to do would be to murder me. 
However, he lives in the building, has a known antagonism toward me, and has a very strong motive. He would be the first and quite possibly the only suspect in my murder, and therefore it would be a big risk. OTOH, it’s very unlikely that he would be suspected of murdering my parents, especially considering that the smartest people on the planet had so much trouble figuring it out.
Good one!
A woman is driving a large truck along a high mountain road. She comes to an old wooden bridge that crosses a steep-sided canyon. The bridge will break if she drives on with the truck, but she can’t turn around as the road is too narrow and this is the only road that can take her where she needs to know. She stops the truck, gets out, and sees a herd of goats coming up behind her. She instantly knows how she’ll continue forward without any danger. What does she do?
Shouldn’t this puzzle have a wolf and some hay in it?
When the woman successfully crosses the bridge, are the goats already on the other side?
In.
I’m going to revise my answer to “kind of”.
Are only some of the goats on the other side?
Do the goats that cross over do so by some other method than crossing the bridge?
Embedded.
Someone’s been enjoying the wonderful Futility Closet podcast! 
Love it! Been a long time listener.
Do the goats actually walk across the bridge with their feet touching it?
Is the number of approaching goats so large that she can destroy the bridge, herd them all into the canyon, then drive across the pile of goats?
Can she attach the contents of her truck to the goats, enabling them to carry the goods across the bridge and also lightening the truck sufficiently to drive safely across?
Or, is the contents of her truck dead goats, so she can unload these, follow the live goats across the bridge on her truck, then kill the goats and reload the truck?
No.
Embedded.
If the goats’ feet do not touch the bridge, then does the truck have a lift of some sort that she can use to lift the goats to the other side?
Is the fact that she’s a woman of any significance?
K. Not vitamin K, just “K”. 
Will the bridge break because the truck is too heavy?
Will the bridge break because the truck is too light?
Is the truck empty before she comes up with her solution?
Is putting goats in to the truck part of the solution?
Is the contents of the truck dead goats?
Is the contents of the truck goat meat?
Is the contents of the truck goat milk?
Is the contents of the truck goat feed?
Is the contents of the truck materials that can be used to strengthen the bridge?