I'm a lifelong collector of a variety of (very) niche things, and have at times sold or tried to sell items from my collections, or whole collections at once. You're right about everything, I would only add that each category of thing is its own world in terms of liquidity and how certain you can be of obtaining a guide price. It also pays off to learn about the collector cultures and communities surrounding each type of thing, so you know what obscure periodical or special interest show, etc. to target when you are trying to sell. Never be in a rush, and the other thing that can make a difference is developing good product photography/videography skills.