One aspect when selling on Cardmarket is that the site/the support does not know their own rules or site structure apart from MTG. You can’t expect any more help as a seller than having a dispute with eBay, don’t expect any knowledge about cards on their part. Additionally, their grading standards are vastly different to any grading company, since “nm” cards may not have a single scratch. Any nm card with a scratch is a valid return case.
I also had around half of the few sealed items I sold over Cardmarket (to new to the hobby-buyers) in the past few months returned because of factory damage (air holes, dirt under the foil, scratches etc) that the support agreed to be valid return cases.
Then there’s also the issue regarding Japanese Cards: www.elitefourum.com/t/update-selling-most-japan-cards-on-cardmarket-is-forbidden/28591/1
Anyway I think those are two helpful things to know when engaging with Cardmarket. Buying PSA cards is the easiest thing on there btw in my experience, since the grading isn’t subjective to seller and buyer anymore. 
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