1-Already in, 2-Interesting and will discuss, 3- Traders that buy everything? No, just no. There is a ton of junk, and unless they feel they can sell what they buy, they got no reason to buy 'junk.'
Investing in traders sounds interesting, though it should be rare and on par with some rather specific situations/quests. A possibility with every trader sounds too powerful, or perhaps just... well. Let's look at it this way, how many people are going to just take free credits from some strange dweller? Too good to be true in their eyes, and it will possibly be right depending on how the player handles things. Might seem like a criminal enterprise and player gets tossed into jail or out of CC for being an assumed front of a syndicate enterprise. Or the trader just keeps the 'investment' and acts like it never happened when you ask about it in the future. Who is the lawkeeper gonna believe in most areas, the trader that helps their community survive or a relatively random under dweller saying he gave money to the guy?
Others are set up to handle investments rather well, perhaps having such options for the Armory in SGS. Maybe it is a cash investment and quest related help in ensuring they got new routes to get the new/better gear, and thus opening up the better tiers and quality of things after a little work and money instead of 'it just getting better' like it does now. The trader that moves into the gang base (spoilery a little so withholding names) would be another perfect optoin for this. He's a visiting trader handling the gangs material needs. Gang has manpower and you can provide investment for better quality equipment. Maybe it takes a 'daily/weekly' tithe to keep it up.
Anyway point being, with minor quest framework, can see all sorts of traders having a myriad of results from investing. Not all will have good results, but variety imo is spice that makes games like this even better. Who knows, maybe you'll inadvertently fund a criminal guild arising in a town and taking it over.....