If the set on auction is relatively new and the box is damaged, then I don't even look at it. Non-starter. If's it's older and the damage is minor then I think about it. I don't know the spread on costs but there are tons of ads on eBay for damaged sets that turn over every month and I don't imagine damaged sets sell very hot on Bricklink.
I have started to message the seller before I bid and ask if they will double box. If they say no, I don't bid. If they say yes, then I will bid and if I win send a follow up email with the information I provided in the message above. I keep a stack of boxes hidden in my apartment for my shipments. Best way to ensure I have a proper fitting box when I need it. And there are actually stores in cities that sell used shipping boxes...fortunately for me it is 1 mile away.
After I win an auction I will send the seller an email with my address and ask them to confirm their shipping cost. If they need more money for the larger double box then I will pay, I just ask to see a receipt of the final shipping cost to make sure the seller is not trying to rip me off.
I am not writing these emails to pontificate but there is certainly ways to ship a mint set and have it arrive as such at the buyer. I agree with capsa77 that it sucks spending good money on a mint set and getting it with creases, it takes all the fun out of buying that set.
Don't get me wrong, I'm not a total box snob, I have accepted sets with damage...they were single boxed. Now I insist on double boxing.