There is a difference between markup and profit.
Markup is how much more than the procurement price your selling price will be.
Profit is the money left over after accounting for all your costs.
Procurement- cost of buying the item
Labour- cost per minute (or hour) of preparing the item for sale- cleaning, photographing, describing, listing, PLUS packaging for shipment.
Fees- as dennis said, listing of your first 50 items is free. You are told before you click "list my item" what fees you will be charged including charges for those options. Also Paypal.
Shipping- is actually paid by the buyer, but there is a 10% fee based on your first domestic shipping fee.
It may seem reasonable to double your cost.
Take an item that you buy for $1.00 and plan to sell for $2.00
If you spend 10 minutes (at 17cents a minute) in labour, your cost is now $2.70.
If it is the 51st item you list this month, you pay another 30 cents and your cost is now $3.00.
If it sells, eBay has a Final Value Fee of 10%. On that $2.00 that's another 20 cents and your cost is now $3,20.
When you ship, let's say 50 grams at letter rate domestically that's $1.20 and the eBay fee is 12 cents, making your cost now $3.32.
And Paypal will charge you 30 cents, making your cost $3.62.
So by 'doubling your money' you lose $1.62.
You need to know what your actual cost of selling is before you even buy the items you plan to sell.
That $1.00 item has to sell for at least $7.50 to 'double your money'. And that's if it sells immediately. On average sell through is only about 40%.
In my opinion, it is not worthwhile to sell any item for less than $9.99 USA/~$13.00CDN.
YMMV.
