I have no discs. Actually, after 12 months, my BluRay drive is still a virgin, no disc has ever entered it.
As for digital, I download, play and sell/trade it, so I only currently have a couple if games on my HDD.

List of PS4 games I have played so far, though:
Battlefield 4
MLB 14 The Show
Madden NFL 25
Madden NFL 15
FIFA 14
FIFA 15
Tomb Raider Definitive Edition
Infamous: Second Son
Bound by Flame
Watch Dogs
The Last of Us Remastered
Metro Redux
Destiny
Shadow of Mordor
Alien Isolation
Lords of the Fallen
Peggle 2
The Walking Dead Season 2
Need for Speed Rivals

The Evil Within

+ 3 games to be downloaded tomorrow:
GTA V
Far Cry 4
Little Big Planet 3

On disc:
-The LEGO Movie Videogame
-LEGO Hobbit
-Plants VS Zombies Garden Warfare
-Diablo III Reaper of Souls
-Knack
-Assassin's Creed IV Black Flag
-MLB 14 The Show
-Wõlfensteín The New Order
-Destiny
-(* Picking up 3 more games I preordered on Tue. the 17th of Nov.)
*Little Big Planet
*LEGO Batman 3
*Farcry 4

Digital Downloaded Games:
-The Last of Us Remastered
-Infamous Second Son
-Infamous First Light
-TRIALS Fuzion
-Peggle 2
-Minecraft
-The Unfinished Swan
-Doki Doki Universe (bought it, didn't download the free PS+ version)
-Resogun (bought it)
-Sounds and Shapes (bought it)
-Fluster Cluck
-King Oddball
-DriveClub
-Flower
-Costume Quest 2
-Stick It To the Man (bought it)
-Octodad Dadliest Catch
-Hohokum
-The Last Tinker
-CounterSpy
-The Swapper
-Oddworld: New 'n' Tastey
-Puddle
-Worms Battlegrounds
-Child of Light
-Pure Chess

That's not counting the free games and still have more on the other devices.
I also have a Vita and a PS TV.

The last of us call of duty

Retail:

-TLoU:R
-Killzone: SF
-Infamous: SS
-Knack
-Tomb Raider
-CoD: AW
- Battlefield 4
-Wolfenstein: TNO

Digital:

-Destiny
-Legend of Korra
-Tansistor
-Resogun                                                                              -Dead Nation: AE          -Infamous: FL

I have the 17 games in my sig, plus a bunch of indie games.

I've also preordered GTA V and FC4.

Is it possible sell/trade digital games on PS4?

Of course it is, I do it all the time. You just exchange account info (username/pass) and that's it...

I prefer going with all discs. Cheaper (20% off at BestBuy + $10 pre-ordering games right now), and of course they are easy to trade.

I would have brought more but I am one fussy bastard.

Killzone: SF
Fifa 14
Knack
Infamous: SS
Call of Duty: Ghost
The Last of Us: Remastered


Digital

Flower
Escape Plan
Every PS+ game