I just wrapped up a big 100 item shipping bonanza on ebay from selling junk around the house. things I learned
-- have the shipping materials ready
-- have a scale (I use a food scale, for weights up to 6.6 lb)
--pack up the material, weigh it in the scale, and put the dimensions into the calculator in ebay to estimate shipping with USPS
--you HAVE to use ebay shipping labels to print. Priority mail gives a 10-20% discount, but then passes the savings onto the customer (ridiculous!). It's close to 30% for UPS.
--factor in the shipping and handling to list price. I walk / ride my bike to the post office so no gas, but the printer ink, paper, envelope, and packaging tape is factored in (I figure .50 a listing).
--I do all costs priority mail, but if the final bid or price was low or less than it should be, I'll ship it to them the cheapest means possible and keep the shipping profits. Some people pad the weight saying it's more than it should be, then reentering it.
--if you plan on selling a lot, search/buy "poly mailers" on amazon, like size 10x13. They are lightweight so don't add much to shipping weight, cost $.10 a bag, and can be folded onto itself to double as a bubble wrap / protector for smaller items.
you definitely have to be careful. There was 2 items that I ended up losing money on because I underestimated the weight, and so the shipping costs was higher than the winning price + shipping.