I found a sealed, never opened Intel 8086 Project Board / System Design Kit in the trash and sold it on ebay for $225.
I bought a used vacuum cleaner when I was in grad school for $30. I used it for 10 years, and then I sold it on Craig's List for $30.
I bought the factory service manuals for my truck for $85. I used them extensively to help fix the truck for about 10 years. Sold them for about $65 on ebay after I donated the truck. Probably the best investment I'd made up to that point in my life.