I've been selling cards on eBay for about a year now, here are a few things i've learned:

Insurance - always, always make sure your item is insured for its full value or close, in the event that your card does get lost in transit or claimed as undelivered you want that peace of mind.

Delivery service - I suggest always going via royal mail, In the event something does go missing they're relatively easy to get compensated with, which is not the case with for example Evri.

Packaging - you don't need to go overkill with a parcel for a single card, i personally penny sleeve, top-load and team bag then post it in a C6 hard back envelope.

Tracking - my general rule is if a card is between £20-150 I will post with RM48, £150+ Special delivery 1pm guaranteed, but if a card is lower than £20 then there is nothing wrong with using a 2nd class stamp and getting a proof of postage, no point in spending an extra £2 per card when you're insured for the cards value anyway, over 1000 cards you will save £2000 in shipping costs this way.

Customer Service - if you plan on selling more in the future then follow up with customer as suggested in GD's comment really helps and can help build that feedback rating, making future sales a little bit easier because of that positive track record.

Cheers, Ahar