Are you selling more than one of a particular comic as different items?
The first step I would do would be to work out which ones were missing. If you know and trust someone who is good with a word processor/spreadsheet, you can ask them to compare the list at: (e.g.)
https://www.ebay.co.uk/sh/lst/active?limit=200&action=sort&sort=-scheduledStartDate
to
https://www.ebay.co.uk/str/garybellscollectablesstore?_sop=10&_trksid=p4429486.m3561.l170197&_tab=sh...
(It's possible the sort order may not be the same and they may have to add all your 800+ listings from several pages of results.)
Another idea would be if you don't have any active listings which are out of stock
https://www.ebay.co.uk/sh/lst/active?action=summary&status=OOS
then to bulk edit all your listings apart from say 50 to be OOS i.e. quantity zero, and then wait a bit for your shop listings to go down to hopefully just under fifty making it easier to spot an example of one which is missing. Note, if you have any listings with available quantity more than 1, you probably will want to leave those alone or keep a separate record of how many you have. You may have to do it on more than one batch of 50 - or maybe use a number larger than 50 - my point is that even if you have to look at 400 it's easier than 800+.
Once you find an example of a missing item, you may notice something about it yourself - or you can just ask ebay why item ######### isn't shown on your shop. You could ask in this thread additionally if you like. You may still not get the answer either way!
(And then bulk edit your listings back to the correct quantity.)
I could do the word processor work (quite quickly) - but you would need to send me the page contents somehow and it obviously contains data which is "private" for the seller. Hopefully you know someone who could do it.
