Vjylku

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.

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"

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.

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)

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.