My “little niche” for buying storage units was the furniture. I was always looking for storage units with good solid real wood furniture. The real wood would allow me to easily fix anything broken, paint or re-stain if needed, and at the very least provide me better return on my money.
My theory to support this is, Las Vegas, where I purchased most of my storage units. Las Vegas is a real transient place. Everyday people are always moving in and out of Las Vegas, therefore made used furniture a safe bet.