I’ve been meaning to slim down parts of my collection and focusing more on my main collections for some time now. (mainly for storage/convenience reasons)
I was wondering how you guys do it? Mainly looking at my fellow Europeans 
How is ebay for eu peeps? Is cardmarket a good way to sell raw cards? What about a website of your own to sell?
Thanks
Just speaking from a German perspective, so the following might apply more to European countries than the US, but that’s what you seem to be looking for:
Ebay is still your best bet. It has the best reach and you are relatively well protected as a seller. Inform yourself in advance about the paperwork you have to fill out if you ship an item internationally (what you have to fill out will depend on the destination country and on the price of the item. I also would advise against underdeclaring the value of your sold card, but others seem to be more relaxed about it.
What you really have to watch out for is deciding whether you want to be a private seller or a commercial seller. That’s important because (in Germany) you have to pay taxes on sold cards if you are a commercial seller. Depending on your overall income, that can be quite a lot. Things get problematic if you try to sell as a private seller to avoid taxes but the tax office sees your doing and classifies it as commercial. That can happen if you sell many cards or actively promote your items, either by being an ebay power seller, having your own website to sell cards or even by making your ebay profile look professional. IF the tax office classifies you as commercial, you will have to pay taxes for your sold cards retroactively, so you really want to avoid this.
So (in Germany), you have to choices:
Sell as a private seller (in which case you shouldn’t sell more than 10 - 15 items per year and also you HAVE to wait at least one year after buying the item in the first place before selling it again, otherwise you will have to pay tax on it as well)
Sell as a commercial seller, in which case you have to register as a business and pay taxes for each sale
Once more, what I said applies to Germany, but I could imagine that there are similar regulations for other European countries.
Ebay works great for me as European. You can choose to only sell within the EU if you want to avoid the hassle of international shipping (outside the EU).
Cardmarket is primarily for lower-mid tier singles AFAIK. If you consider going that route, you have to ask yourself if that’s worth your time. I see many listings there for cards worth a few cents. Personally I couldn’t be bothered to package a card and go to the post office for such little money.
Other than that, do you have any specifc questions?
Thanks for the very helpful answers!
I think I’ll look into the ebay route and also look into private vs commercial selling here in Belgium
I am using Mercari for my low cost stuff. I will use Ebay when I’m selling higher priced stuff.
I’m a filthy yankee, so things might be different on the other side of the pond, but I think piecing out your collection on eBay is the way to go. It takes effort and it’s admittedly an overwhelming hassle at times, but if you care at all about optimizing your profits, I think it’s well worth it.
I decided last spring that I wanted to consolidate my collection, unloading a lot of low-value stuff to fund some high-end purchases and fill in holes in my personal collection. I’ve spent a lot of my free time during quarantine going through boxes and boxes of old cards, mostly WOTC through BW-era English set cards, and listing them on eBay. I currently have over $10k of cards listed, and fewer than 10 of them are over $100. I’ve also sold about that same amount in the past year, and the most expensive card I’ve sold has been around $100… Pretty much all the money I’ve made has come from selling lots and lots of cards in the $5-$50 range.
You’d be surprised at how much you can make by piecing out your stuff on eBay if you do it right. Of course, eBay and PayPal absolutely fuck you on fees, and you have to make a decent investment in shipping supplies and postage, but the reach you get from eBay is unparalleled. On eBay, you can eventually sell pretty much anything, even cards you’d probably assume are unsellable. I’ve been able to sell tons of stuff that most people would consider bulk, like EX-era rares and BW-era holos, for like $6 per card. It really adds up. Also, unless you urgently need cash, I’d recommend listing cards for more than you think you should; don’t be the cheapest listing. Somebody will buy it eventually.
If you’re selling higher-end stuff that tends to be more liquid, try selling it on Virbank first to avoid the vicious eBay fees.
Generally ebay is the best but if I was selling high value I’d much prefer to do it in person unless it was someone well known and trusted.
I hated selling on Facebook groups because I would list something at 80-90% of market value and get offers of 50-60% market value