Shipping costs have sky-rocketed in the 3-4 years I've been thinning out my train collection here on the forum.
All of the major shipping outfits have online price calculators that are good -- providing you know the correct weight and size of the package, along with the destination zip code. If you have the original shipping carton, developing a shipping quote will be easy and straightforward. But shipping can get involved when you need to pick-and-pack different items you're selling -- especially if you're selling individual pieces of rolling stock and somebody wants to mix-and-match different pieces. Nine times out of ten, the box you want to use will be off by a mere fraction of an inch.
Don't assume shipping costs... take the time to calculate an accurate quote using the online shipping calculators. If you shoot from the hip and do a quick estimate, you'll ALWAYS come up short. Been there more than I care to admit in my early days. So now NOTHING goes out the door unless I have a firm understanding of the shipping costs.
Best of luck with your sales!
David