Thanks! That’s very helpful. I also ran across this article. I can’t vouch for the author and some of his economics sound a bit iffy, but in this paragraph he offers a much more compelling reason for a return to barter (and one that’s the polar opposite of the official spiel I referenced earlier…).
Hadrian may have temporarily halted the economic crisis that Trajan’s policy was rapidly creating but beneath the surface problems persisted. This manifested itself within the currency in particular. With the strain of foreign demand and the steady movement of currency eastward as a result of the adverse trade balance evasive action was taken. Nero took the move to contaminate the denairus and to reduce coins by clipping. The weight and quality of the denairus fell constantly until the time of Commodus when inflation was reaching cataclysmic proportions. In his reign the silver denairus sank to one third of its former value, and wholly ceased to circulate outside the Empire. The aureus became so unreliable that by 200 A.D. it had ceased to be accepted abroad without testing for weight and quality. The story of the third century was one of worsening inflation and the minting of bad money. The situation became so bad that many people fell back on the natural economy of bartering thus further perpetuating the economic problems.