You get 50 free auction-style listings (which can have a "Buy It Now" option too on there) a month. You do not get charged an insertion fee for those. If you JUST do a Buy It Now, it has an insertion fee. If you list it in multiple categories or have special features (they offer the gallery for free - so you get 12 pics as standard) there are additional insertion fees.
But, in general, you can list it for free.
When you sell it, you will be charged;
9% of the total value of the sale
and 9% of the amount you charged for shipping.
Pay attention to the last one! They calculate shipping for you, but do not include the extra 9% in there. So, you may wish to put in a "handling charge" for certain things or just be aware of it. Alternatively, you can over-estimate the weight of the item.
PayPal also takes its pound of flesh; they want 2.9% plus 30c (in the USA).
So, the final calculation to work out your profit is;
( (Final Value + Shipping Charge) x 0.881 ) - ($0.30 + actual cost of shipping)
I strongly recommend the use of an online printing service for the shipping - eBay has one linked to their site, or Stamps.com is very good too (I use that for work as well, and is a really easy interface). Online shipping is cheaper than buying it at the store, and you can get tracking and insurance right there. You can also pay the eBay service directly with PayPal.