By this point, you're probably noticing the word 'vintage' come up again and again on our list. There's a rich and wide market for nostalgia. People love to re-connect with the things which made them happy when they were younger, and they'll pay a high price if you have an item which can help them do that. That still applies even if it's a niche item like an old microphone.
There days all microphones look similar - the world has agreed on what a microphone 'should' look like, and the Shure SM58 is about the industry standard. Decades ago, though, they came in all sorts of weird and wonderful shapes and sizes. It's the oddly shaped ones that go for the biggest money, like the Neumann U47 from the 1940s, which has a guide price of $10,000. Was there a musician in your family? Were you a musician years ago? Do you still have any of your old equipment in storage? Head down there and take a look!