Can I enter cost information for items that have already sold?

You sure can.

If you go to the Inventory -> Sold view, you will see a list of items that have sold, sorted by date. For any items where Seller Ledger couldn't automatically match cost information, you will see a link called "Add costs"

Click on this link to bring up options for adding the cost information:

Now, there are 3 options for adding cost information:

Option 1: existing inventory

If you have already entered cost information for that item into Seller Ledger as inventory, buy we were unable to match it automatically (for example, if you didn't use the same unique SKU in Seller Ledger as you did in your listing,) you can simply choose the item - it will be listed under a sub-heading called "Available stock"

Option 2: prior purchase with no details

If you don't have that item, with its cost information already in Seller Ledger, but you have categorized all of your prior inventory purchase transactions, you can choose the purchase when you acquired the item that just sold. From that same list, you will see all inventory purchases that do not yet have all of the item details under them.

Choose the transaction in which you purchases the item that just sold, and you will be prompted to enter the cost of that item.

Option 3: no history of any purchase

In the situation where Seller Ledger has no knowledge of any prior inventory purchase that could contain this item, you will need to add it. To do so, choose "New Inventory" and enter information about the purchase of that item:

Note: option #3 will create a manual inventory purchase, just as if you had manually entered it previously and chosen that item from inventory. This option works well if you are manually tracking expenses within Seller Ledger, rather than linking bank or credit card accounts. If you do link a bank or credit card account that could, in the future, import a purchase than you have manually entered as inventory, you could wind up with duplicate transactions.