Well its simple, you don't tell them to do HI. HI is not the only way to make money.
They get free level 120 sea jewels. They can farm low level sea instances every day. Its not easy and fun, but it can earn a decent amount of silver if the player loots everything and sells to docker. It wont earn as much as HI, but for a newbie, it will be enough to get him started.
They can also do some actual merchanting. High level players are often buying exploration items since they are lazy to farm them themselves, items like beliefs from drifters, holy judgement ores from gibraltar, St. Mary figureheads, Refined weapons etc. They are still buying badges from London daily. They are still buying Moonstones, Bloodroses.
Heck, there are players actively buying level 140 alts for Portobello, newbies can make those alts and sell. Yeah it sounds kinda stupid, selling alts, but you'd be amazed how good it can be. There's lots of things high level players need, and newbies have access to. They just need to be willing to do the "dirty work".
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