handy resource: https://www.usps.gov
You can calculate what postage is going to be from and to somewhere if you are pretty close to the weight of the package. Usually, a single 45 packaged in a Bags Unlimited box with fillers will weigh about 5oz and cost $4.80 from where I am (Atlanta) to the UK. On ebay, I charge $6 for international air mail because that accounts for more expensive shipping to Australia/Japan from here, and if it goes to UK/Europe, the slight overage will help pay for my mailers and is not grossly over the actual charge to the point where I would expect anyone to complain. For my set sale customers, I charge $5 for international postage for the first 45.
I'm guessing that most sellers have their own "system" for coming up with their postage costs, honest or dishonestly - and I would guess that challenging a seller on this wouldn't go down very well after the auction has ended. Best advise is to ask any questions of the seller before bidding ends, and avoid bidding if you aren't happy with what she or he is offering.