Honestly, I'm not selling anything, and I've managed to get around 4k caps in over 20 hours. I've got a lot of weapons, armor, etc. that I can probably make 10x that amount if I took the time to do it.
But advice, check every container, and also if you've put some luck in, take the Fortune Finder perk.
You need to invest into your settlements.
Settlements:
-Trading posts produce income over time (this is pretty marginal, even with emporiums (level 3 trade huts) I barely see 500 caps per 1 or 2 days. You can collect the caps from your workshop.
-Build a bunch of water purifiers (Sanctuary has a good spot for this) and let the water collect over a few days in your workshop. Take the water and sell it.
Alternative means:
-Make food and sell it
-Make chems and sell them
-Improve weapons and sell them
Do NOT break down prewar money into cloth, you can find enough clothes for that. Prewar money is worth money.
Also, If you are low on caps and want to buy something expensive, consume graqe mentats and equip clothing that boosts your charisma when you are ready to buy it. The price will be much cheaper, particularly if you have a low to average charisma. You don't even need the chemist perk to make the graqe mentats. You can find clothes, headgear, and glasses that increase charisma and wear them at the same time. This will also boost the resale value of your stuff if you change clothes when selling it.