The Cardmarket webshop has a FAQ answer for this.
How should I pack my order? If you are selling 1-16 single cards, you should:
- Put the cards into sleeves (max. 4 cards per sleeve)
- Seal the sleeve with adhesive tape.
- Use a piece of paper between tape and the sleeve opening to prevent the tape from touching the cards.
- Fix the sleeve(s) on rigid cardboard (e.g. a postcard, but not "normal" paper).
- Use another piece of cardboard to "cover" the cards from the other side.
You may of course use a toploader, but only 1 card per toploader. And you need cardboard in addition.
Send in a normal envelop.
Bubble envelopes are not a suitable protection for trading card
shipments. We do not recommend to use bubble envelopes
For larger orders, we recommend to use piles of cards that you wrap
into paper (like a present ;-)) to form a block. Make sure to fix and
protect the block in a box or big envelope.
Using the FAQ like someone else but adding a few things.
If using a toploader, place card inside a sleeve, place sleeve upside down in the toploader and tape it, tape toploaders together if sending multiple cards.
Tape the toploader to the envelope! I can't stress enough how this makes a difference, I had some cards arrive the other day (through the breach, engineered explosives and a noble hierarch) that the envelope was shredded to bits on one side but the cards were fine.
Always ensure that the type of postage is adequate, if you are sending 0-20$ worth of cards, use postage that covers that amount, you may risk without it as the risk-benefit (cheaper price) might be worth it for you.
If you are sending individual cards worth $50 or more, make sure the insurance will cover this
This might sound stupid but...don't use the money you got from the cards until you are sure they have arrived safely, the last thing you want is to have to refund $300 to someone while you wait for insurance (or not) but you are unable to as you already bought something else with it.
Be smart, keep safe!