I find a box to ship the item in, pack it, measure it, and weigh it. I plug that info into the eBay shipping calculator and it tells the buyer what it will cost for shipping. As a seller, you get a small discount, but I am not ashamed of that. I have to buy bubble wrap, tape, paper and ink to print labels, gasoline to make a special trip to the Post Office, and some shipping boxes and envelopes that I had to buy. I cry a little inside when I put a kit on eBay and see what the buyer will have to pay for shipping, but I can't control what USPS charges. I sold a large scale kit a while back and the shipping was around $90. Was an odd size box, and it cost me $12 for a shipping box that I had to modify to ship the kit in.
In all, do your homework and be an honest seller. I have had two flakey buyers that eventually paid, but I immediately blocked them from ever bidding on any of my stuff ever again.