My question is related to / inspired by Does Colombia require proof of onward travel? but isn't quite the same.
I'm thinking of a trip around South America (from Europe) such that:
I would not arrive and leave the continent at the same country. Instead, I'd arrive somewhere in the south of the continent and leave from the north, or vice versa.
I wouldn't buy flights or overland journeys for within South America in advance (because I'd prefer leaving exact travel dates and routes open).
For example, say I have (only) two flights booked in advance: one from Europe to Argentina, and then, 2-3 months later, another from Colombia back to Europe. When arriving in Argentina, or any of the subsequent countries (e.g. Brazil, Bolivia, Peru, Ecuador), would the return ticket to Europe from Colombia be enough to prove onward travel, or would they require a ticket out of that particular country?
In other words, is a return ticket to Europe from another country sufficient proof of onward travel in most South American countries?
NB: I'm interested in South America generally, not only in Argentina or Colombia. But of course, if there are countries that are more strict about this than others, that would be good to mention in an answer!
How do people normally do this? I mean, it's a very common type of (backpacker) trip to make, right?
I guess the main alternative is to book all border-crossing flights / buses before going to South America, but that limits your flexibility a lot (or is a waste of money if you buy them but don't use them).