You can credit old content in the gallery. I do it all the time. The only issue is if the item is no longer being sold. If an item has been removed from the store, you aren't going to be able to credit it anymore than you can credit something that has never been in the store.

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Available on Amazon for the Kindle E-Reader Monster of the North and The Shimmering Mage

Today I break my own personal record for the number of days for being alive.
Check out my store here or my free stuff here
I use Poser 13 and win 10