Play Asia and limited run games are producing limited amount of niche games that get sold out fast depending how small production they have. Buy a copy and sell later when demand is high so you can triple your money. Often its some weeb game that has fanservice. Mary skelter series for switch/vita is one that always triples its value after they are sold out. It costed around 40$ and now is sold around 150-200$ on ebay. Companies usually tell in advance how many they print out, so just pre order when its small amount and limited amount of copies.