Set up your own traders in the settlement.

Note, I've never bothered selling water either ... but then I've never needed caps for anything.

Everything is so plentiful that I've never needed to buy a single thing

You cannot play this game like a console jrpg, where you buy and sell things and then end up a millionaire in the end. I never buy anything from these cutthroat vendors and if I do, it's usually ammo, and i offset the cost with some drugs or other valuables. These vendors are not selling things because it's fun being a trader, they do so to survive, and will rip you the hell off every single time, so avoid dealing with them and almost never buy from them unless it's a unique weapon.

Install http://www.nexusmods.com/fallout4/mods/2832 mod. It is not worth the effort to do water farming without this mod.

I usually just sell junk loot and ammo i don't need. The water purifier stuff just isn't my cup of tea since i dislike the settlement side of this game personally. Not to mention setting up your own vendors requires investment into perks that I don't really need. best advice I can give when trading for expensive things you want like shipments of crafting materials is to wear your best charisma boosting clothing, either the suit or dress you get from rescuing Rex Goodman in trinity tower, hat such as the militia hat and black rimmed glasses for 5 charisma, then eat graqe mentats. You can make the mentats by having 2 hubflowers (found in most field expanses in the game) a bottle of whiskey and of course regular mentats. This will boost your charisma another 5 points on top of giving you a flat reduction in overall product cost. Drink a beer too if you have it for another +1. Just vendor everything and trade it towards the item you want, I usually loot every bit of ammo I can find as well as weapons. I tend to leave armor behind since it eventually becomes very weight intensive for a mediocre return in value. Don't focus so much on accumulating physical caps since they are largely worthless and by default most vendors do not traffic in enough caps to give you for your items outright, so always barter just enough to cover what you want from them in equivalent value.

Scrounger perk my friend. With the first tier I never fall below 20,000 caps and that's without selling anything. Ive got 15,000 lbs of loot to be sold just sitting in a chest in sanctuary because I haven't needed it.

Water farms, mate.

5 industrial water purifiers in Sanctuary ≈ 200 purified water per day ≈ 2500-4000 caps per day (depending on CHR stat, perks and bonuses from magazines).

I can buy whatever, whenever.