Undo delete? You mean like if you throw out a unique/untradable item? Isn't there a 2nd confirmation that basically says something like "This item is unique/untradable and may not be reobtainable do you really want to delete it?" that you have to click yes on to make sure you aren't throwing it out on accident?

You do have immediate buyback if you sell it to a merchant provided you didn't either close the transaction screen with the merchant or sell enough items to it to push it off the list because I think it only displays up to something like the last 20 items.

Undo delete basically means making another hidden inventory list for every character that exists to store deleted items for people to "undo" later.