How much adjustment? $100, $1000?
I dislike wasting a seller's time. It would be up to them to give me some sign that they were interested enough for me to stick around for a bit. They know to their satisfaction what they have, it's not worth itemizing flaws, other than for myself.
I am not asking how you would dicker with the seller, but simply, how much less would you be willing to pay? For me, assuming there is not already another repair (seems somewhat more likely than the mysterious left-side crack), this crack would probably barely scratch the top dollar I would be willing to pay. Maybe $100-200 at most.
I think your question only works if it is for sale and a price is set. If it's a beater Smith on a used gun rack for three hundred dollars, would an identical crack have the same consideration process? I don't think such a small figure is worth mention, seeing where the bidding currently is at and where it may go.
I'd bet if a dealer wins this auction, then turns around and retails it at a hefty mark up, that might revise the value of the crack. It's all relative, based on the seller's optimism and the buyer's motivation. Most folks wouldn't 'discuss' an auction they were participating in until after it was over? I couldn't put a dollar figure on any bumps and worts until cash is actually changing hands.
