One of the most underrated factors of sending a card is making sure the card will not shift around in it's top loader during the journey. Worst case scenario, shifting can cause a card to partially come out of it's top loader, in the process turning a BGS 9.5 (Pro-Mint 10) into a BGS 4.5 (Pro-Mint 9).
Sellers/traders often use tape that requires a surgical approach to extracting a card or destroys a perfectly good case - both of which can easily be prevented with the use of painter's tape. For those not familiar with painter's tape, it's essentially masking tape with an emphasis on ease of removal and the absence of residue.
The nice part about painter's tape is that it's cheap ($2-$5), provides the exact amount of protection needed, doesn't require scissors to remove (scissors & cards have never seen eye to eye) and doesn't ruin a perfectly good card case.
Also, be sure to pinch the top loader together where the tape is applied to keep the card from shifting around during the mailing process. Painter's tape also works perfectly with magnet cases, snap cases, hinged boxes and pretty much every form of card containment.
Related Topics: How To: Selling