When listing an item for sale, you are given several postage options.  For instance, how much Media Mail will be, how much First Class will be, how much Priority Mail will be, how much Express Mail will be.

On an eBay search, the DEFAULT is to show the FIRST shipping option selected by the seller.  If this is, say, Express, it might look like you are trying to charge $15 shipping for an item, even though you are also willing to ship First Class for 39 cents.  People who are scanning through listings might not even look at your item, assuming you are gouging on postage even if your postage is reasonable.

This is especially true of shipping calculators.  eBay will now calculate postage based on zip code and display that amount in certain searches.  However, I have seen many instances where Express is selected as the default shipping method even when it is one of several that the seller is willing to ship, and there are less expensive methods for shipping.

If you are not sure what postage is showing on your listings in an eBay search, it is prudent to run an eBay search for one of your items and see.