Part of what drew me to this hobby was my dad collecting bikes when I was a kid, and my favorite ones were the ones with the horns/horn tanks. When I got into collecting, I wanted all my horns (and lights) to work.
Between upgrading my collection to better, more desirable bikes, and the fact that I take the batteries out for storage, and I don't ride them that much anyway, it just got to the point where it wasn't worth bothering with anymore. Now when I buy an accessory that runs off batteries, I'm more concerned if it's correct for my bike than if it works.
These old lights don't always have bulbs and even if they do, they might not be any good. I'm not going to search for a bulb so I can try it. I'm also not going to bother putting batteries in something to try it, just to take them back out again, or go buy batteries that I don't have on hand. Nor will I try to figure out how to make something work, that takes obsolete batteries (like a teacup light that runs off a battery can)
If I sell something as untested, it's because I honestly haven't tested it, and don't care to go through the hassle of doing so.
Some people, like me, don't care if its tested, untested, working or not working, we just want the part to complete a bike. Some of this stuff is hard to find in any condition. If you must have a part that's guaranteed to work, you're going to spend a lot more time searching for it.