If it were, say, a Parker 24 gauge, then it would be worth a whole LOT more. But that would be because of rarity and the collectible factor. If you're talking about a gun that doesn't have a "name" or something else to make it particularly collectible, and if you're planning on using the gun for hunting, then 24ga is a pain in the butt. You can get shells from Fiocchi, and a lot of people think "neat idea, half way between a 20 and a 28!" However, all 24ga guns, as far as I know, are 2 1/2". Standard load is 5/8oz if I recall correctly. So you have a gun with a bore that's half way between a 20 and a 28, but the shot charge is less than a 28.
Unless you really like it and want to fool around with a 24ga, I'd probably try to get them to take it back. But as mentioned above, wouldn't surprise me if they covered themselves in their auction policy by putting the burden on the buyer.
