You get 40 free listings per calander month. any over that you pay a listing fee even if it is a relist,  it will show you at the end of the listing before you upload it how much it is going to cost. there are extras to pay for too, for instance listing in more than one category. You also pay a final value fee on every item that is sold. It is extremely rare for ebay to get the fees wrong so you should check your invoice to see exactly what you are being charged for.