No problem @dariogalleno.
You can map custom meta field values to existing item specifics provided by eBay, but you cannot use custom meta fields as custom item specifics on eBay, only product attributes.
To map custom meta field values to existing item specifics browse to the edit product page or listing profile (make sure a primary eBay category is set) then scroll down to the “item specifics” section and locate the “Pull value from attribute” column on the right.
Click the drop down selector and select the custom meta field you wish to use in the section titled “Custom attributes”.
Kind regards,
John