I had a ton of stuff to sell and was almost at the weight limit (Dogmeat full) and ran into the Brotherhood of Steel on the way and they wanted to go grab some transmitter I guess, in a robot-infested building, real quick so I agreed. Basically this was an escort mission... for me. Paladin Danse just slaughtered robots that can kill me in two hits by the dozen, so I just let him go while I picked apart each preceding room for loot. I didn't kill a single thing during this mission (which was for the best as I had no health and >50% radiation). Had to trudge from Cambridge to Diamond city without the ability to sprint. Look at my HP bar when I arrived, the entrance was quite a welcome sight:
http://i.imgur.com/FZLb2fL.jpg
Found a bazillion more things during this little mission so when I was done I was about 150lbs over the limit of almost pure profit. I've also barely used any ammo. Plus I've found lots of caps from luck and talked NPCs into giving me lots extra during every quest. Hilariously, Piper's "con" way into the city involved faking that I was a trader with a huge shipment of supplies. Lol...
Basically I can buy and sell everyone in the city many times over but they don't have the funds to buy my wares. How long does it take for them to replenish their store of bottlecaps? Realized Dogmeat doesn't come into the city with me and he has also got a bunch of stuff on him too.