Seems as though most people don’t understand economics in a video game but I’ll try to shed some light.
In an MMO when you kill a monster, currency appears. This currency regardless of the source is then in the system until removed by an outside source. On its own the currency is worth less however it’s value is decided by a few factors such as the amount of time it takes to obtain amounts of currency as well as the cost of fixed items in the game such as silver sinks which effectively work to stop a literal infinite supply of a currency.
This is different from real life because irl the only (legal) way to obtain currency is to exchange items or time for it (a job ect) and that money never fully “leaves” the economy since money doesn’t just materialize like in an MMO. Thus irl it’s much harder to reach a state of inflation (value of currency comparable to amount of currency ) than it is in an mmo since again in an MMO money literally appears from nowhere. Because of this if this money isn’t going into silver sinks it inflates the prices on the server . This gets worse because RMTing is rarely done though players but through bots who don’t need to eat sleep or go out. Because of their ability to constantly generate silver and trade it for money that isn’t related at all to the game, this throws the economy into a death spiral. This is different from tp since the value of those items is 100% decided by the community and isn’t in it’s self it’s in form of currency. You can’t buy anything at all in the game for a diamond anvils, however you can trade it to another player or sell it on the market. Thusly TP trade doesn’t actually hurt the game(in the framework of this discussion, the tp shop is a mess and that’s a topic for another day).
Tl;Dr: RMT bad for a multitude of reasons. Tp trade is bad but not for the same reasons.
