Back during the summer sale I sold some dupes and bought the ones I needed, but I spent like 20 minutes making sure that the amount I spent was equivalent to the amount I got from selling. Other than that, I've just sold the cards from games I didn't want to get the badges for.
I bought one summer card just to complete the set - I can't be bothered with trading. I sold off any duplicates and a foil card during the sale - And all earned cards for a couple of games with only 5 cards in the set - They sold for more. The rest just sit in my inventory being mostly ignored. (I say mostly, as I was given a M&B: Warband booster set. I would have sold it but its practically worthless, so now I have more worthless cards than I did earlier).
I look at the price of the summer cards occasionally still though - Mostly to laugh at those who thought they'd be worth more post-sale. Even the foil cards have dropped in price by a decent sized chunk.