I believe it could go a number of ways as far as obtaining a house elf. Some may be bought, some traded, and some handed down amongst family members. I think either of those scenarios are possible and quite likely to happen.

It does seem that elves so quickly and willingly offer their services, but I also think the elves see it as their way of living---their only way of living. In all honestly, what else is out there for an elf (I'm sure there are many potential opportunities, but that does seem to be the popular view most wizards and elves seem to have)? They don't live in the wild the way a number of magical creatures do, nor are they kept as pets and cared for by humans. I think, in some ways, they see this as their way to have a dwelling area, as well a means of having shelter, a minimum amount of food to eat, and a purpose for living. I think they see it as "all that's out there" for them. It is very unlikely they will survive a revolt, and it's equally unlikely that enough wizards will agree to set them all free to create some other sort of life for themselves.

I also see no other way of the species surviving and carrying on outside of mating and having descendents. We've never seen evidence of "baby elves" roaming around, but they've all got to come from somewhere at some point. I think their young are raised to do exactly what they do and then carry on that way of living, as well.