Whether you list your item via the site or the API, it makes no difference on when the item starts showing up in searches.
Here are the reasons why the item does not show up:
If are use the API, ensure that you set the ItemType filter correctly to get all the items that match the query. If you do not set the ItemType filter, it may not return the stores items depending on the site. For instance, if you are using the Finding API call findItemsAdvanced, set ListingType to All.
If an item is listed on a site with the shipping available just for that site, then the item will not show up in the searches of other sites. For example, if an item is listed on Ireland and the item is shipped just within Ireland, the item will not show up on US (www.ebay.com). Similarly, it will not get returned in the response of findItemsAdvanced for SiteID 0 (US). It will get returned only on www.ebay.ie and if the SiteId is set to 205 (Ireland) in the API call.
If you have any additional questions about your item not showing up, please contact our customer support team.