Gold Bars are worth about 450 caps at full price but even then your netting around 200 caps even if your horrible at trading.
Anything with Gold in it is worth money.
As said earlier Pre-War money weighs nothing but is also usually worth more than 1 cap each, plus it tends to accumulate at the workshop when your dumping off stuff.
Pure Water is easy to produce at Sanctuary when you can build quite a few water purifiers along the River, Heck I built those plus a nice Water Front fort too. Built another Water Fort at another Settlement Location with like 3 more top end Purifiers too just cause it looks cool. Any settlement location with a large standing body of water can potentially earn you quite a lot of tradable water and there are several locations to use for this purpose.
Sanctuary of course is tempting to plant a sizable farm on due to the large amount of dirt on this location for planting crops. The best crop to plant is in fact Melons due to the Abernanthy Farm being so close by and since a certain young lady is willing to buy your melons at about 3-5 caps each. You could plant quite a large amount of Melons on Sanctuary to produce a substantial dailey income if that's your thing.
Not to mention that in general having a large farm is a great way to produce an income if you need to. Having a decent amount of CHA helps due to the settler limit being CHA + 10 more settlers means more farm hands so you want to get as many people farming as you can.
Charisma is a big factor in pricing and I'd bet the usual character build that has 1 CHA is well why even bother trying to make caps or trade for anything...Kinda pointless to even bother doing anything with Traders with 1 CHA.
Just one point of CHA is quite good, consider drinking a Beer before talking to venders and often Trash Can Carla has a few beers to sell so you could buy one, drink it, and then sell your loot. Gotta say though if your into selling and buying loot you probably should invest 6 points into CHA right off the bat.
Magazine Perks there are 10 mags for 10 levels of free perks. A barter perk with 3 levels you can buy at level up.
Altogether though a starting character just isn't that good at bartering and characters that don't invest in barter are not that much better either.