Even if I just sell some used items in the bay I get loads of strange questions and requests. If you engage with them then they may bid, if you don't then they probably won't. It's a trade off, do you need their interest or are you better off avoiding those buyers anyway? It's a personal choice.
I think the challenge here is you aren't just selling a used piece of furniture, you are selling something you have crafted yourself, with a label in it that says 'I made this'. At that point I think a lot of people will feel that they can ask you questions as they would as if they were commissioning an instrument from you. It would be different if you were just selling a bunch if used ukuleles I imagine.
In addition (and quite rightly) you are selling the instruments above cost (and why not, even as a hobby!). But when you do that you do become a business in the mind of the purchaser. When you shell out a lot of money for something your expectations of the accompanying service do change I think, you do feel entitled to ask a few questions, get some reassurance etc.
That may suck, but it is what it is.
