I have only sold keycaps, but I have looked at keyboards on r/mechmarket too and also have heard from others about the aftermarket. In general, anything that is a limited run like a GB will hold its value because there are a fixed number of units. Sometimes it will increase in value, especially immediately after the GB has delivered. Even something that has more than one round will hold value since it's usually a long time between production rounds.
This has been true of keycaps in my experience. I was able to sell a couple sets for exactly what I paid for them after owning them for a few months and realizing I either didn't like them or just wasn't using them. I probably could have charged more, but I don't like the feeling of cheating someone. My experience has also been that keyboard stuff sells VERY fast. Every time I have sold something on r/mm, I have had multiple offers within minutes. In fact, the response is so great that it has made me procrastinate selling more stuff because I hate the offer and counteroffer that happens with multiple people and keeping track of who was first and such.
You have to be a bit careful if it's something that will eventually go into regular production or be hosted on a store's webpage. If something is generally available or easily accessible, its value decreases.
