Hi.
I will answer your 2nd question first: There are two different fields for postage. One is for the shipping that you charge the customer, the other is for the postage that you pay to ship the item. If you print your postage through ebay, it should pull in the correct amount for both of these fields. Also, you can manually change the postage value (that you paid) to whatever you like directly in the sold listing tab if needed.
For your first question I think I need to find out more about what you want to accomplish. Did you want to avoid putting these item purchase costs directly into the sold list after they sold. Perhaps you want to record all of it up front instead of filling it in as it sells?
If thats the case you can fill them in the product master (even though they are one time selling items) with the item description (ebay listing title) and purchase price (and anything else you want to automatically fill into the sold list when the item sells)
Let me know if thats not what you were after.