I am going to be asking a series of questions that will be relevant to forming some sort of a picture of human space commerce.
Let's say that Earth-based human civilizations have discovered a series of ancient jump-gates that allow them to travel within a large and varied interstellar network.
There are not many clues, apart from the jumpgates, as to who left this system behind. For the moment, I am assuming that there is no bias to the kind of systems included in the network: i.e. its not like systems with earth like planets make up the majority of the planets in the network. So, "system types" have roughly the same probability of occurrence as if one were just taking a cross-section of space and scanning it.
Put another way, the gates simply connect a large number of close-by stars, rather than a large number of only useful stars.
While genetically-engineered humans exist in this "universe", no non-human sentient aliens have yet been encountered.
Also, jumpgates do not limit the size of the ship that can be transported through them.
Oxygen is needed not only for breathing, but also perhaps for making water, with hydrogen. While recycling can do a lot, there is going to be inevitable loss. How easy would it be to gather oxygen in space?