Online sites for auctions and trading such as eBay will charge a nominal fee for listing known as a listing fee or insertion fee. The listing fees that are put in place by such sites will depend on the value of the items that you have for sale as well as the number of days you wish to keep your item in the auction.
The higher the values of your item, or the higher the minimum price you will sell it for at auction the higher the listing fees. The listing fees will also be increased if you wish to take advantage of a priority listing.
On some sites, such as eBay, the listing fee can be waived for a certain number of monthly listings. This can help you get more exposure without cutting into your profit margins due to listing fees. Ebay also refunds the initial listing fee if the item sells, but then they have an additional final value fee, which is charged on all items sold.