You might want to obtain the Merchant trophy first so that you have 90k remaining. There are a few ways to obtain 100k.

Sell any unwanted mods, weapons, Warframes, Sentinels or Sentinel weapons.
Same as with the Merchant trophy.
Play Alert missions.
These are special missions that pop up on your main screen occasionally that have increased payouts and sometimes even rare items as a reward. You can only complete alerts in levels you've unlocked, but you can hitch along if others invite you.
Play Invasion (Operation) Missions
Each faction will regularly invade missions. You can see these missions on the right side on the main menu. Press :triangle: and choose any of the operations. You can also look them up on the Star Chart. Most of these missions have a high credit payout as a reward. Pay the required number of missions and wait until the invasion has completed. Once the invasion is complete, no matter the outcome, you'll receive your payment via your inbox. (Note: The invasion isn't complete once you've completed the mission the required amount of times. These missions need to be played by the community until the progress reaches either 0% or 100%. This can take a while if both rewards aren't interesting enough.)
Play Void Missions.
Special missions with a high payout. These require special keys that can be found in Survival/Endless Defense missions. You can also use the chat to find anyone hosting a Void mission.

If you don't own many trophies from the main list just yet, simply play the game and you'll earn credits for each mission you'll complete. Don't worry too much about this, since it will easily come along the way. If not, once you do own most of the trophies from the main list, you'll easily have enough items to sell.