A soft plastic sleeve (like the ultrapro or dragonshields used when playing a deck) in the mail is not enough support, and cards shipped that way that I have received
have come in damaged.
The best way to ship is in the rigid plastic top loaders, after that between two pieces of cardboard or poster board, something more rigid than the card(s) in the middle that will protect what you are shipping.
Almost all cards I have received are one of these two ways, and none of those have ever been damaged during shipping.
If you use cardboard or poster board, you need to tape the pieces together, so they do not slide apart and the card slide out during shipping.
A single piece of tape over the opening of a top-loader can also help, particularly with some top loaders that are larger than others.
There is less risk of the card sliding out of the top-loader than there is of it sliding out from between two free floating pieces of cardboard, but the risk is still there.
As for what you ship them in, any envelope card size or larger will do.
Magic the Gathering cards are Poker* sized cards are 3.5" by 2.5" and top-loaders are 4" by 3".
The smallest envelope according to USPS rules that qualifies as a letter (for standard stamp usage) is 5" by 3.5" so anything around that size works perfectly to fit a top-loader.
*
Yu
Gi Oh cards are slightly smaller Bridge size cards at 3.5" by 2.25"
boardgames.stackexchange.com/users/22772/Andrew
In addition to Andrew's answer, which is 100% correct, you can write "Do Not Bend" on your envelope.
I have heard that this will tell the post office not to put your envelope through the sorting machine, which can bend or fold your envelope, and hence damage your card.
Usually your card will be safe even without the "do not bend", but for extra safety you can add it.
boardgames.stackexchange.com/users/24528/Ertai87
I would like to add, its always better to ship valuable items with tracking and insurance.
So a normal USPS letter wouldn't be the best.
Maybe use something like FedEx, UPS, DHL and so on.
Its more expensive but way more safe* (*citation needed)
boardgames.stackexchange.com/users/26521/undefined
I usually put cards in soft plastic sleeve like ultrapro, and use bubble mailer instead of normal envelope.
For more expensive ones I prefer to use rigid plastic top loaders.
You can also add some cardboard for better protection.
Few layers of paper also work.
You also should make sure that cards will stay in center of envelope and do not accidentally move.
Bubble mailer is thin on side where it is sealed and offer less protection there, so cards should not move in this area.
Another thing to keep in mind that people who are stamping envelopers may hit it hard with stamp, what may damage card inside.
Because of this card should not be directly under post stamps to reduce this risk.
boardgames.stackexchange.com/users/24427/Daniel Frużyński
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.
Very important: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!
boardgames.stackexchange.com/users/26278/fireshark519
The Cardmarket webshop has a FAQ answer for this.
Let me just copy it here.
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.
boardgames.stackexchange.com/users/2804/b_jonas
Is there an agreed upon standard on how cards should be packaged and shipped when they're sold online on places like Cardmarket or similar websites? I have some valuable cards I'd like to sell, but have never done so before.  Do I simply ship them in a sleeve with an envelope? Or is something more secured the standard? We're talking single card sales here, so each card is out on the market individually.
How should I ship cards?
boardgames.stackexchange.com
<magic-the-gathering>
boardgames.stackexchange.com/questions/45165
boardgames.stackexchange.com/users/26513/Gentlemoth
Use a rigid plastic top loader with two pieces of cardboard on either side. Write "Do not Bend" on your envelope. The best way to ship is in the rigid plastic top loaders, then between two pieces of cardboard or poster board. Use a piece of paper between tape and the sleeve opening to prevent the tape from touching the cards. It is also always better to ship valuable items with tracking and insurance. Another thing to keep in mind people who stamp the envelopes may hit it hard with stamp.  Bubble envelopes are not a suitable protection for trading card  shipments. Send in a normal envelop and write on 'do not bend'.
The best way to ship is in the rigid plastic top loaders, then between two pieces of cardboard or poster board. Use a piece of paper between tape and the sleeve opening to prevent the tape from touching the cards.    write "Do Not Bend" on your envelope.   its always better to ship valuable items with tracking and insurance.  Another thing to keep in mind people who stamp the envelopes may hit it hard with stamp.  Bubble envelopes are not a suitable protection for trading card  shipments. S11: Send in a normal envelop.   