i think this is because in aincent times, people where branded slaves got brands in almost any culture, in some cultures they also branded thifs...., the writer used this and embelished it.
if you dont read it litilary you alredy have a number, your social security number, your tax number your account number ..... it is kinda hard to legaly buy and sell whit out them.
yea the same money all over the world is a bit impossible say an african child that woks for 1000 xy a month that equals one euro a month if the world currency would be euro how would the child spend it cause the change is not small enough for their prices. and if you fix the money so that we al use their xy one would need a truck to bring enough money to go grocery shopping in our country. so the same money all over the world is impossible whitout the same standard all over the world, and that aint gonna happen.
as for the worlds economie to go broke it can happen if someone can turn led in to gold it would happen fairly quickly since moste currancies are based on gold on some level.