Another person has said they found people will pay more for a camera with the box on eBay. That is probably a unique situation because it's sight unseen from an unknown seller. Having the box might give them a sense of security. I wouldn't pay extra but other people are different.
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Tom
Look at the picture, not the pixels
I think it's much more subtle than that...
Your description makes it sound like there's 2 options to a single sale from the same vendor...you can have the item without box for (say) $500, or you can have the item and the vendor will throw in the box for $600. In that situation, most people probably wouldn't opt for the second option, because it's obvious then that they are paying $100 just for a box.
In fact I think it's more likely to be the situation where vendor A is selling an item without the box for $500 and vendor B is selling the same item with box for $600
All other things being (apparently, as this is Ebay) equal, many people (perhaps myself included) would skip over the listing from vendor A and only bid on the item from vendor B.
Yes, it eventually amounts to the same thing (I'd be paying $100 for a box), but that's not the point. Would it make me feel I was getting a "better" item because the box is included? Yes, probably. Could I rationalise that? Probably not. Would I be prepared to pay extra for that irrational, warm & fuzzy feeling? Yes, likely I would, up to a point. Could I put a number on that? No, probably not...it's just a feeling.
It's not rational, clearly. But buying something never is rational, that's why there's a whole industry built up around advertising. There are always weird little foibles going on in your decision making, some of which we are aware of, some of which we are not. I would never, ever buy a white car, despite how close it might be to my desired model/options. I just hate white cars, and I'm aware of that. Other purchaes, I just know I'm drawn more to one item than another, even if the performance is equal, even if it costs more...and I can't always say why
You might not have such foibles over buying secondhand camera gear, but I'd wager you have others...
On a slightly different but related note (because I'm not at all sure it affects second hand camera sales the same way)....try selling a collectable toy/memorabilia/whatever at auction, and see how much more you get if it's in the original box...