Packing the cards:

Place the cards gently inside a top-loading baseball card sheet or top loader case. Make sure the cards are inserted properly and do not shift around inside the top loader. Top loaders protect the corners and surfaces of the cards.
For multiple cards being shipped together, use ultra-pro or similar brand top loaders for each card and then place the top loaded cards inside a rigid cardholder page or binder sheet. The pages have plastic sleeves that hold the top loaders securely in place.
Make sure to use acid-free or acid-neutral pages and holders. Acidic materials can damage cards over time.
For very rare or valuable cards, you may wish to use additional protection like putting the top loaded card inside a penny sleeve before inserting into the page. Penny sleeves add an extra layer between the card surface and top loader.
Take care when packing the cards into pages or binders so the edges and corners of cards are not bent or damaged in the process.

Packing the shipment:

Use a rigid mailer, like a bubble mailer or cardboard mailer, and not just an envelope when shipping cards. The rigid package protects the cards from any bending or crushing that can occur in automated sorting machines.

Include at least 1/4 to 1/2 inch of bubble wrap or air packing material on all sides of the enclosed card pages/binders for padding. More padding is recommended for more valuable cards. Bubble wrap or air packs cushion the cards against any bumps or drops in transit.

Do not overstuff the mailer as this can lead to crushing, but make sure all empty space in the mailer is filled to minimize shifting during transport.

Peanut/popcorn packing pellets or shredded paper may also be used for lightweight padding and filling empty spaces.

Seal the package securely with packing tape, wrapping the tape completely around the mailer such that no edges are loose or peelable. This helps prevent empty space inside from shifting contents.

Clearly write “Baseball Cards” or “Trading Cards” on the mailing label so postal workers handle it appropriately as a non-rigid package rather than an envelope.

Consider shipping with tracking and insurance for valuable cards being sold, as this provides proof of delivery and coverage if the package is lost or damaged in transit.

Shipment drop-off:

Use your preferred carrier like USPS, UPS, FedEx which all generally have insurance options if needed. Avoid general delivery mailboxes as they do not provide true tracking.
Consider signature confirmation at a minimum for expensive cards, or require adult/direct signature depending on the sale value, to ensure the right person signs for the package. This offers more accountability than basic first-class mail.
Double check the address is correct before leaving the package with the carrier. Make sure to get verification like a scan or receipt showing the package has been accepted by the carrier.

Feedback and communication:

Message the buyer when the package is shipped with carrier name and tracking number for their visibility into the shipment process.
Request delivery confirmation and feedback once the buyer receives the cards. Ensure they arrived safely and there are no issues. This helps your seller reputation.
Be available via eBay messages after delivery in case an issue comes up like damage to address concerns promptly.

Following these steps will help ensure baseball cards are securely packed and transported to their buyer in the same condition they were in when shipped. Proper materials and quantities of padding as well as using tracked shipping protects seller and buyer. Good communication during the process adds accountability. Adopting best practices is key for maintaining positive online marketplace experiences selling collectibles like trading cards on sites like eBay.