These days trading cards can be worth quite alot! I should know since i'ce been collecting Yu-Gi-Oh! Cards for a few years now, and have bought ALOT of cards through ebay both domestically and internationally.
When it comes to packaging cards properly there are a few easy and relativly inexpensive ways to go about it, probably the most popular is using Toploaders. Toploaders are cheap medium-hard plastic cases designed to transport trading cards without damaging them. They can fit most sizes of trading cards from Yugioh to Magic: The Gathering to AFL Football Cards. They normally cost within the range of $5 for a 25 pack, so its only 0.20c more to add to the shipping costs, and in my opinion thats a small price to pay to secure a mint condition card worth up to $20. I personally use the method of inserting the card into a protective soft slip, Then into a toploader to ensure absolutely nothing will get to the card.
Another method i see quite alot is people cutting out two pieces of cardboard and sticky taping the cards between the two pieces. This is also very effective but can be time consuming, and you need to use hard but flexible cardboard (cerial box cardbox isnt the best...), so by the time you find the materials you need and cut it all out (if your selling alot of cards then it can be a pain) you couldve gone and bought good quality toploaders for a few dollars which you can charge your customers the extra few cents for. Your customers will appreciate this alot more than tightly taped cardboard (NOTE: many customers, given their age, can end up damaging their cards by trying to slit open the cardboard/tape with a knife or scissors, and when they do that they most likely arnt going to blame themselves)
So have a good think about it before you just throw an exposed card into a thin envelope and hope for the best.
I hope this helped you somewhat.