Melon farming? Selling items with a few perks turned on? (Or is that slow because you can only get what shopkeepers have in their cash box?) Rob a bank? What's the most efficacious method for getting rich fast?
The best way I've found to earn caps is to become a moisture farmer.
Every unit of purified water sells for 7+ caps (more if you have the right perks), and its possible to ramp up your water production to the point you are making over 500 units of water per 24 hours.
The best part about it is that the starting town, Sanctuary, is the perfect spot for it, there is a nice and long section of water you can easily fill with multiple water purifiers.
The only thing left to do is to gather these rather common ingredients:
Industrial Water Purifier:
- 4 Oil
- 2 Ceramic
- 10 Rubber
- 4 Copper
- 20 Steel
- 4 Cloth
- 6 Screw
- 3 Screw
- 3 Gear
- 7 Steel
- 3 Rubber
- 3 Copper
- 1 Ceramic
After that, its just a matter of avoiding this glitch:
There is currently a glitch in Fallout 4 where the production stops if there are already units of that item in your workshop, so you need to go back to town every once in a while to take out all the purified water and put it into another container.
Here is my Sanctuary water production facility:
Don't forget to add defense to your town too, it takes 5 heavy machinegun turrets to match the production of 1 industrial purifier, gathering the materials for that is actually more annoying.
Also, if you set up some traders in your town you can carry your water over, dump it all for their inventory of ammo and gear without having to go to diamond city.
Finally, for a second and more easily defensible water production facility, Kingsport Lighthouse is a good choice.
Crafting the common chems (e.g. Jet and Buffout) and selling them is an easy way to get loads of money fairly quickly. The resources required for Jet are Fertilizer and Plastic, which, while disgusting to picture in your head, are easy to get or cheap to buy.
Next, you sell your Jet stash to the vendor of your choice and get all his caps. If you still have Jet left, travel to the next vendor. Repeat this ad nauseam. If you don't want to travel after exhausting a vendors cash, you can also 1) build your own vendor town in one of your settlements with a load of vendors packed in one spot or 2) trade your Jet for his ammo. Either you need it to kill stuff or you can trade it for something else later, which is convenient since ammo has no weight.