It's a can of worms, but I'll give you the biggest worms.
1. If those sellers are in China or Japan, there is a service that is contracted with USPS to provide insanely cheap shipping costs for them to ship to the USA.
2. If the seller is in the USA, they most likely have selected "flat rate" cost for all buyers and have calculated that shipping this item, at it's current weight and size, will cost (for ex.) $9 to ship across country. So they charge $9. If the item ends up selling to a someone in the same state, it will end up only costing them $6, and they pocket the $3.
3. Some sellers will select "calculated shipping", which tells ebay "go ahead and display an estimated cost to the buyer of what it's going to cost to ship this to them at their location, considering the weight and size information I have given you". If the buyer is across country, the estimated shipping cost will be much higher for that buyer. As opposed to a buyer who lives in the same state as the seller, the cost will be lower.
My best recommendation (option a), without knowing the size and weight of your items, and without knowing who's gonna buy it, and from where...is to do "standard shipping" flat rate. Ship your item using "ground advantage" service from USPS. When you are listing your item, box it up as if it's ready to go, (label the outside of the box so you know what's inside), then weigh the item. Google "ebay shipping calculator", select "ebay rates", input the information, and towards the bottom you will see "select major city". Choose a major city far away from you and see what the estimated costs are. If the farthest away price is $14.99, then go with a price like...$12. In my experience, depending on what the product is, people are willing to pay $15 shipping for something that weighs 3-4 lbs. If the buyer ends up being closer, you win. If the buyer ends up being at the farthest point, AND your item is heavier than 4 lbs, it may cost you $20 to ship. But if you're willing to eat the rest of the cost, then your chance of selling the item faster will increase compared to a seller who has "calculated shipping" selected.
My best recommendation (option B) is to look into USPS priority mail flat rate options. You can use their boxes and/or envelopes for free (pickup at your local post office or ship to your house via their website). If you can fit it in the box, it ships, as long as it's not over 75 lbs, for a flat rate.
If you're selling on facebook or craigslist or on here, use pirate ship dot com.
