How to handle cards?

It depends on the value. Sometimes i sell them, sometimes i craft badges, sometimes i transform them to gems so i can craft booster packs. If you're not interested in crafting badges the best bet is to sell or trade them of course.

yea alot of people just sell em but some collect em (like me :3). Got around 14500 cards atm.

I like to craft badges with the cards sets I have for games :).
The only tip I could give you regarding this is that you should not craft your badge when you have all the cards but keep it for the Steam winter/summer sale when they award you with a seasonal card whenever you craft a badge. So you can get a few extra cents for your pocket through doing it then.

3/5 cards in a set I buy the remaining 2 for 3 cents each if possible. If they are more expensive I prefer to sell the 3 cards I have.
3/6 I sell or attempt cross-trade with other 3/6 set. Same applies to other ratios, really depends on the price of cards.
It's not worth to turn them into gems mostly, if you have unmarketable cards it's better to try cross-set trade first.
And of course if you want to craft badge, wait for summer/winter sales so you get an event card to get some profit.

What I usually do is:

• Craft badges : Set containing 5 cards and are cheap (less than $0.05)
• Sell cards: Everything else which meet above criteria with few exceptions.

I mean only you can answer that. If you like collecting cards then collect them. If you want to trade them for something you need then trade them. If you think they're a waste of time then just sell them as soon as you get them.

I sell every card I get as soon as I notice I've gotten it, and I usually have a handful of euros in my wallet every steam sale to pick up some nice game for myself. Those few cents here and there adds up nicely.