Now that 3d printing has advanced, we could potentially print replacement parts to requirements when we need them at the location. A Moon colony would allow easy refuelling and launching, so so would a colony on one of Mar's moons. I remember reading about one of them being highly porous: underground caves provide both a location for a base and radiation shielding for any inhabitants. I wonder how much gravity mar's moons have? You'd have to supplement them with centrifuges, of course, but a little gravity is surely better than none.
Water can be mined and used to both refuel spacecraft, restock the Mars/Moon base and send it back to earth as pure water. We're going to need more water soon, and desalination isn't going to cut it.
Ores as well, of course.
The thing is, with enough investment, these things become cheaper enough they become worthwhile. With all the resources of space for us to take... what couldn't mankind accomplish?
Not really, the moon has significantly more gravity than a NEO, and a comparatively lower water content. (Ie, less avaible fuel).
As for Mars's moons, they're just captured asteroids. You can reach orbital velocity using a bike and a plank.
And sending back water to earth is not going to happen. Especially not from the moon. I'm pretty sure that you'd end up wasting more energy on sending it up (and down again) than you'd get from desalination here. I mean, even the most energy expensive desalination installation uses a mere 25 Kwh/m³. => 9*10
7 Joules
The lunar escape velocity is 2.38 km/s. Resulting in an energy need 283*10
7, just to get the mass of the water in orbit.
There, it's physically impossible to come up energy positive.