English is not my first language, so expect some eccentric grammar and wording.
Here’s the thing. The prices in the game don’t make any sense. Well, maybe they do in the modern day, real America, but not in the world of Cataclysm. Merchant dude just sold me a bunch of 9mm rounds for the price of dirt under my nails. Is he still living in the bygone age of functioning assembly lines and non-zombified neighbors? Because that’s the only possible reason why the bullets and guns in popular calibers are not treated as a new gold. Everyone who is still alive and even remotely sane will be hoarding them and very hesitant to sell them. Prices will go up. Way up.
By contrast, rare and cutting-edge firearms will become dirt cheap, almost unsellable. Every scavenger wants the box of 9mm, or .223, or .45. RivTech guns will be forgotten - too high-tech for the new, cruel world without overblown military budgets. Caseless ammo is finicky, hard to produce and impossible to re-use.
Old and obsolete guns aren’t much different. Merchants just won’t carry (that is: usually won’t buy or sell) any ammo for the old Cattlemans, because in pre-Cataclysm world it was produced by the handful. Precisely because of its relative rarity, it must be cheap (right now it is 4 bucks or more per bullet, jeez louis) - there will be no demand for it, if your character is not a hardcore Saint-of-Killers wannabe.
I can give many more examples, but my vocabulary is depleted. To summarize: the barter economy in Cataclysm currently reflects the realities of modern consumer society, not the survivalist society. Merchant NPC’s are like retarded magpies; they give away the good merch in exchange for useless, but rare shinies. Rebalance is necessary.