Ok, It looks like I'm going to have to play devil's advocate on this one.
For starts, you need to consider the seller is not a vacuum collector. With that in mind, you have to give the seller a break.
Secondly, about the bag itself. It is probably old, and vintage. I don't know about you guys, but I have not seen bright yellow bojack Kirby bags on the market in recent years.
Third, the untrained seller noted that it has a "K" on it, and it is possible that this bag was attached to a Kirby. I think the seller used this kind of information to come to his incorrect conclusion.
Lastly, a few words on rarity. I'd imagine this is somewhat rare. Have any of you seen one in this exact style? I have seen plenty of bojack Kirby bags but I don't recall seeing one exactly like this. Just because it is bojack does not make it common.
I think this seller has a vintage, rare, and nearly worthless bojack Kirby bag.