All you need to do to sell is go to you inventory, click on the card and click "sell". It'll show you a line graph of what price the item has been going for lately and you can put in what you want for it. Steam always takes a cut (10-15% I think?) so it'll show you what you'll end up getting too. The snow globe cards will all disappear when the sale ends so the price is really crashing.
I changed my mind and got The Walking Dead Season 2, so now I only need one more card. If I don't get it tomorrow from voting, I'll probably just buy it since it's only 10p atm.