I cannot stress this enough, that not having this is a HUGE pain when trying to determine whether I have all of my items listed or not.
When I look at my items in game, they are sorted alphabetically, but I can’t match that up with the sorting in gameflip.
The result? I always have a large number of items not listed on GF, because it is simply too tedious and time consuming to type in each item name to see what I have listed, or, to scan down several pages of items and try to spot them all visually.
I don’t know how others feel about this, but for me, it’s a royal pain! 
If someone just has a small number of items, it’s no big deal, but when you have hundreds or more…it really matters.
The good news is, this should be easy as pie and super quick for your designer to add. They could probably do it in just a few minutes tops!
Please consider adding this right away. I have partly given up on listing everything because it’s such a pain to keep track of it all. I have too much stuff! I want to expand my listings, but the lack of this feature, plus some other lacking features which I will suggest in other threads, make it a real chore.