Hello, I remember helping you once before when you were asking about your convertable car, right?

lol well. Firstly, go to the post office in your town and ask them about how much it would cost, write down the prices for all options such as normal, fast, overnight etc types of shipping. Then when you put the item up for sale on a website, give all these options for shipping and their prices. So for example lets say overnight shipping is 60$, you want $100 for the item your selling, you would list the overnight shipping as 60$ and the item as 100$ then when you get the paypal payment or w/e style of payment your using, just withdraw the 60$ and use their money to ship it and pocket the $100 for yourself.
Edit: And NEVER ship the item before you get the payment, and if the payment is a check in the mail, CASH THE CHECK BEFORE YOU SHIP THE ITEM!!! and if for some reason the check bounces, keep record of this in case legal issues do arise because some bastards pissy about not getting an item that they couldnt pay for in the first place.
2nd Edit: And you might want to get prices for international shipping too, unless you plan to restrict shipping to just the country you live in, which if you restrict it to only your country make sure you make note of this on the item's info.