You should be doing a pre-authorization for the item that you are selling. This pre-auth guarantees you the money for a X amount of days (varies on issuing bank). Once you have shipped the item / ready to, you can do a post-authorization / sale.
While most companies complete the sale in one transaction - it is usually ok. The card associations frown on this since you are getting money and the consumer is not getting anything. Think of this as you walking into Wal-Mart and buying a TV. Now it will take Wal-Mart another 24 - 48 hours to actually get the money, but you have the product.
When you do a pre-auth, the issuing bank is holding onto that money for you, usually for three days. Most companies will just do the sale to prevent from paying the few cents that it will cost to do the post-auth / sale transaction. As long as you are not doing anything illegal or waiting weeks to ship an item, usually a sale transaction can be done.
If you did do a pre-auth and more than the few days have transpired, you still might be able to do a force ticket through the virtual terminal to get your money. You will usually need some information that is found in the virtual terminal (like transaction ID), but you should not need the credit card number. If though, the few days have transpired and the customer reached his / her limit, the transaction could be denied.
