Since the shops are always depleted of the wasteland currency (maximum I have find during the course of 4 days is 6666 in the Hub general shop, that disappeared within 1 minute, usually there are 0 - 40 caps).
My idea would be:
A scrapper - A vendor that buys basically anything (for caps of course), but pays only half of its base value to keep things balanced. This would solve the problem of having no caps and also the major shitshovelgrind I am forced to do now, simply because I have a lot of loot but I cannot exchange it for money.