If this isn't an item you routinely offer for sale, you may not have marked the "I Sell This Item" option in the Profile view of the Item Information window. Only the items that have been marked as sellable appear in the Select from List window. To sell the item, return to the Item Information window and mark the "I Sell This Item" option and review the account that will be used to track income from the item.
If you've sold the item using the Sales window in the past, it's possible that the item number (or name, depending upon how you've chosen to view lists of your items) was changed inadvertantly. If you think this might have occurred, display the Items List window and click the Sold tab to view a list of all the items currently available for sale. If the item was accidentally renumbered or renamed, highlight it and click Edit to change the number or name.
This tip applies to:
Sales window -- Item layout