Selling them in the most profitable way is a hard one to give a right answer for....I can break it down for you though...It is up to you after to determine for yourself what is most profitable for you :) (what you have the patience for)
In order to get a sack of gems you need 1000 gems and the price RIGHT NOW on them is .73 cents.
Cards vary in the amount of gems they are worth depending on the age of the game, number of cards acquirable without additional purchases, and rarity of the card (there can be 'foil' cards).
Selling the cards individually usually only net you a few cents a piece and they usually sell at different values in the same way as it would work for the gems. Take into consideration for popularity here, too. The card below has a decent value, while most will sell for around .08 cents. Brand new games can get you card values at about .35 cents if you get them quickly.
Additionally you can sell cards by the pack. You can do this by purchasing packs with gems. The amount of gems a pack will cost you varies from game to game. These packs seem to sell on the market for about the worth of a sack of gem depending on the game, but the packs may cost more gems than the total in the sack of gems.