No game that has seemingly unrestricted trading is balanced - ever. It will be mired with exploits, grifters, bots, dupes, and people who just play the market.
Thankfully, drop rates are not balanced around this fact and taking part in this “economy” only serves to reduce your experience. I love when people RMT all their stuff and realize there is absolutely nothing to do once they max out.
D2, for its time was great but man did it have problems. D2 practically invented duping. The bad part about D2 was that drop rates for so abysmmal, unless you had a handful of builds with Map Hack and Enigma with full MF gear, you probably weren’t going to find anything unless you traded.
Regardless, this is why players have asked for a SSF mode, or really, just Self Found Mode, where you cannot Trade but drop rates are increased.