Taking the Jury Rigging perk is a bottomless goldmine for caps if you can get your Repair skill to 90.
Repairing a .44 Magnum or 10mm SMG to full condition nets you about 2500+ caps and you can do it with a brand new .22 silenced pistol from the Gun Runners that costs around 100 caps, or with the proceeds from your scavving.
Same with clothing, repair high value metal helmets ( 200 caps ) with pre war hats ( 6 caps ), repair Combat armour with NCR etc.
It's a little bit of a gamey way to get money but speaking as a scavenger that has never left so much as a bent tin can behind it is the most profitable imo.
I now end up buying high value things if they are in very poor condition from the vendors and end up selling them the repaired version back at full price.
Ah, you beat me to it. Jury Rigging is so ridiculously overpowered.
Other than what WastelandRevival said, keep pool cues and sledgehammers, use them to repair super sledges. Machetes, switchblades and such to repair combat knives. Also save any piece of metal armor you come across, because they repair power armor. Not as good a weight-to-caps ratio as the higher-tier guns but still a nice chunk of caps.
Seriously, in my current game I have in addition to more than 20K caps a full set of unique weapons (some GRA) and all the implants at level 20 or so, and that's counting the five levels or so I spent in Zion where there was only one vendor. And I'm sure I'm behind the curve on this. I don't think any other way of making money comes close. Maybe casino trolling, but I've never tried that.