well... there is no other best way than to invest and sell
but also try to do daycare services to players... like ev training, if u are quick enough, u just invest 30k to fully train certain pokes in safari kanto (or 0 investment if u do it outside of the safari but takes aloooottt longer time to do)
and you can charge players for that services, like 75k to 100k around for ev daycare on certain pokes
(red prices)
If you don't know what you're doing, and you don't have pre-existing income, you can't afford to buy low level pokes and resell them trained. Guaranteed money comes from doing bug catching daily, doing dig spots, and doing daily reward quests.
Donating cash for CC :) Some do, even so with little income 20k etc, and luck, u can place effort to start to buy and resale, i say that from my own experiance as a newbie with 14k, i bought a furret for 5k epic ivs, and i sold it for 50k, it was my first sell, that tells me if someone else can, then others can as well, but again, it takes luck and effort
btw that furret was sold untrained xD there is always a way around to does that bust their butts to work