Cardmarket is great. I made some purchases there when I was studying in Spain.
Hmmm, yeah, going to the post office is a hassle in this situation. Maybe you could announce only the higher value cards, as you said, with the caveat that you will only sell them all together. If no one bites, I would recommend waiting until the end of the pandemic, yeah.
As for buylisting, I can't really give you a recommendation - never did that before XD I've always bought/sold my cards as singles, and only ever acquired collections from friends. I should really start messing with buylists, but I have no incentive currently, since I will be playing only Pauper for the foreseeable future (I've decided to take a break from Legacy until - and if - Wizards sorts this mess out).