Without knowing the mind of your potential customer (or at least the first one that might possibly go for the rest of the deal), I can't really be definitive,
but...A bright colour would suit (UK) city riding, because (along with absolutely every flourescent bit of clothing you can find) you want to be as visible as you possibly can so that the motorist concerned can't
possibly resort to the
SMIDSY defence.
Admittedly, a fluorescent frame isn't going to add much to your copious (or lacking) hi-vis vesting... Also, you're not in the UK (I gather, from your use of the non-u word "color", and I know a number of countries (Dk, De, Nl, Be) are particularly good with cyclists).
(I'm also not a militant "retake the streets" cyclist. I've been cycling for about four decades on the British roads and am far more conservative (small-c) than the cycloactivists that tend to make themselves a nuisance over here, and I
am the kind of cyclist who will stop at lights (even pedestrian crossing with no-one crossing them!), and I don't like using pavements, even "cycle-pathed" ones.)
My bikes over the years have been a couple of different reds, and various blues, of the ones I remember. Relatives bikes have included a subtle (not camouflagey, but not bright) green, orange, yellow.
I think it's odds on whether a good, straight primary/secondary colour (pigment primaries and secondaries: red, blue, yellow in the former case, leading in the latter to green, and orange, but not
so sure about purple...) is going to be a selling point or boring.
I don't like the idea of grey although (assuming you can do it with few enough blemishes) more
silvery greys might be attractive. Chromey would be nice (but need a lot of cleaning.)
"Paint it black and it'll go faster, if not by itself" (a British joke, harking back to Chris Boardman's olympic track triumphs, a few years ago, on the black carbon fibre frames) might work, but for the same reason as with some of the greys I wouldn't personally like it. However, it
could be stylish. (Of course there are those that know that muck shows up more on a nice glossy black than many other colours... But I'm also not suggesting you paint it mud-colour!)
However, when talking about pigment primaries, it made me wonder about the optical colour mix... Cyan?
To the right person that could be quite attractive. And the beauty of eBay is that "the right person" (or, ideally, a couple of them, with the willingness to tussle a little on the bid price) is quite possibly going to pass by with the right site-placements (and a little luck), just as much with one colour as (most) others...
Have I just concluded that it
doesn't matter what colour it is, though? No, I bet it matters, but probably a little less than a lot of other configuration options, which I suspect you're already on the top of anyway.