There are already several warning messages that indicate when that a user hasn't logged into the site in a while, and that the album might no longer be for sale.
Collection page
Sell page
There are a few more spots that I can add the warning. It never occured to me that wantlists would experience the same problem, but I can add this feature to those too.
EDIT
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Ok, done.
Also, if a user hasn't been logged into the site for more than 2 weeks, the notifications aren't displayed when you view the Collection page.