When selling on Cardmarket, there are different problems that arise than when selling on eBay etc, since:
- Cardmarket chooses the shipping method you have to use, often not the appropriate one (especially for card amount).
- Their grading scale is MUCH harsher than anywhere else, no scratches allowed on NM cards - selling a card as Mint is usually feedback suicide as any flaw qualifies for a refund.
- You usually don’t have photos when listing “bulk” like cards since uploading them is a hassle.
- You can’t comment on feedback - if you get a couple bad apples in a row, it looks really bad and hurts your account, especially if you don’t sell a lot and those people stay in your latest 100 sales for a long time.
Now, of course grading is subjective which is an inherent problem between sellers and users on that site and users are entitled to give you feedback based on their subjective grading opinion which is totally fair, but if you ever had people give you “0% 
” because they bought 20 cards at 2€ total, Cardmarket wanted you to ship it in a 50g, <1 cm PWE but the buyer expected 20 toploaders to fit in there, you probably wish that person wasn’t able to buy from you in the first place.
(Or if you ever sold multiple XY evolutions cards at anything above Exc, rip lol.)
TL;DR: Do you think a Cardmarket list of extremely troublesome users is productive/do more people want this? Maybe not necessarily as a block list, but to check if that specific person is especially picky in any way.