The Saboteur

if you set it to freaking hard and blow your self up with RDX you loose most off your ammo for all guns go to dealer restock rince and repeat faster than unloading your guns it nets up over 2000 each time and takes less than a min.

Once you have the required Contraband. Set your difficulty to "Feckin Hard" and kill yourself with explosives. Return to HQ. Purchase all ammo, then kill yourself again. Repeat until you have spent the 75k.

If you still need to earn enough Contraband to sell 75k worth, here is a quick way to get some. (You should note that if you want to 100% complete the game, you will gain enough contraband through normal gameplay of clearing the maps of all Nazi towers, AA guns, sniper nests, etc.)

1. Go to the Champs Elysees area. (I did this after I had completed the main story)
2. Locate that large "round-about" on the map with the design of a sun/star in it.
3. Directly to the SE there is a large building that is off limits and will cause nazi to attack when you are in the area. Go to the South side of the building and use the elevator to reach the roof.
4. In the SW corner of the roof, there is an AA Missile turret. Once the enemies in the area are killed, jump on the turret and look to the sky for a Zepplin and shoot it down.
5. After shooting the Zepplin and gaining contraband for it, spin the turret around and find another Zepplin to shoot. You can continue to spin the turret and find more and more Zepplins that respawn when you look away. I found 2 to 3 zepplins in every 360 degree spin of the turret.
(A tip from PiCkLE SQU1D: Fire twice at the Zepplin then start turning before it blows up. It seems to make the Zepplins spawn faster and will cut down on your time.)

I am guessing there are other spots where this could be done as well, but this one works for sure. This would only work if you havent destroyed that AA Missile Turret yet. If you have destroyed the turrets on this building, I would try another turret in the Southern part of Paris (Area 3) since there tend to be more Zepplins in that area.

*If this solution helped you in some way, please vote positively. If you think it needs improvement, please let me know how in a private message or by leaving a comment.*

I've found a better and faster way.

Go around, blow up stuff, use up all of your explosives (excluding grenades). Then Die, re-spawn at the HQ.

When the game reloads, head over to the nearest smuggler and max-out the ammo for every thing. Total around 2,000. Then repeat.

Another Option:

Get the Terror Scope Sniper Rifle and the Panzershrek and go to the smuggler in Chinatown. Climb on the roof right above him and unload everything. The rifle is auto so you just have to hold the button. No one can spot you, in most cases, and if they do there is a safehouse shack right there to reset your alarm level.

When you unload all of your ammo for these two weapons and throw all of your grenades, drop back down and buy ammo for 860 each trip. I usually don't bother with the dynamite and RDX because it takes too long. This way is a bit more boring but it does keep you out of trouble.

I just bought the Gold perk for sniping and it didn't affect my achievements.