The major problem is that there is no reason to buy much of anything other than to save some time. Everything in the merchants inventory can be found out in the world or made by the player themselves once they get the correct perks. I had the same issue in Skyrim. My solution in Skyrim was to add mods to the game that had unique items you could only get from merchants. Even then, once you had bought everything you wanted, money just piled up and had no real use after a certain point.
Skyrim had one other thing, that made money useful, that FO4 is missing. Player homes you could buy located all over the map in different cities, with different environmental settings. Since FO4 only has two environments, wasteland and the Glowing Sea, that isn't much of a concern. Then there is the fact there is only one real city and it does have a home to buy. Plus we can build our own homes in settlements in locations that cover areas all over the map. So what worked in Skyrim probably won't work here unless you put a new spin on it.
Some unique homes that you could purchase would make a nice money sink in FO4. By unique I mean something we can't build for ourselves like the giant robot statute in the Galleria. Or a plain old water tower like you find at Concord turned into a home. Probably the most popular would be a bunker complex. Based either on a military bunker or a vault. Heck, do one of each.
One of my favorite ideas would be a mobile caravan/convoy home that you can move around the map to selected locations and set up in an empty parking lot. An empty semi trailer or a flatbed semi trailer with operable cargo containers on it would make a great workshop for all your crafting workbenches. A bus or a Armored Personnel carrier for your personal quarters. Yeah yeah, I know, none of the vehicles run any more. That could be taken care of by lots of Brahmin, or some reprogrammed Sentry bots to pull them around. You could then use the Sentry bots to guard camp while your out and about.
It could even be upgraded, for a price, by adding in some of those little house trailers like you see in Fiddlers Green that are staffed by some merchants thus turning the whole thing into a traveling market. At least a doctor and a general merchant.