To some people, "vintage" is just a fancy word for old, and judging from the way it's used on eBay, perhaps a fancy word for "unwanted junk" as well ... it reminds me of how some people eagerly seek to buy "faux" items, not knowing that the word means "fake" ... another favorite, which was in heavy rotation a decade ago is "virtual", which means "not really".
Regardless of what period, though, getting back to audio, I think the collectible or interest in older audio products broadly falls into three (well, four) categories:
There have always been some remarkable products that stand out from the rest. Some can stand on their own as audio devices judged against contemporary gear, without apology.
Others are remarkable for one reason or another, perhaps as breakthrough products or because they occupy some place in audio history, even though they may show their age in comparison to "good" gear.
And then there is stuff that is sought after more because of nostalgia than any inherent qualities as sound equipment, at least in terms of it standing up to the best of today taken as a whole.
The fourth category, of course, is junk. Mr, "Vintage Realistic AM/FM receiver", I'm looking at you, right now, on eBay.
Without going overboard (because you can, easily, with such a list) I'll just mention one or two examples, of what I mean, for illustration. These are just things that popped into my head that I know well; it's not a list of "my favorite" or "my top ten list" or anything of the sort. Just examples for illustration.
My hifi history starts in the early-to-mid 70's ... I know of stuff before that, but I never lived through it, and from where I am now, looking back, I can see that makes a difference.
Anyway ... examples of stuff in the first category; that can hold it's own today ... Quad ESL loudspeaker; Denon DL-103 phono cartridge; Threshold 400A power amplifier; Thorens TD-125 turntable, etc
Second category; historical significance or turning point in audio ... Apt Hollman preamp; TEAC 3440 multitrack tape recorder; Nakamichi 1000 cassette deck; B&O Beogram 4000 turntable, quite a few FM tuners from the 70's ... etc
Third category; mostly nostalgia: The Marantz/Sansui/Kenwood/Pioneer you owned in college, regardless of it's inherent qualities (good or bad); 8-track players; colorful record changers; most mainstream graphic equalizers; etc.
Junk ... well junk is stuff nobody really wanted then, but for some reason people want now, unless of course they were old enough to remember them, in which case they still don't want it and were hoping one of the rewards of growing old was that you would never see them again. Wrong again, I guess.