I would be wary of getting adult beardies, especially females if you plan on breeding. It makes very little financial sense to sell a breedable adult female unless there is something wrong with her. Most of the adult females you see for sale have been bred out or have some other reason that makes them a poor breeder. That doesn't mean they don't make good pets or that you can't pick up the rare breeder, it's just hard to know.

You're much better off getting babies both in terms of quality and knowledge if you are just starting out. It will be hard to help your future customers' husbandry problems (which WILL arise) if you haven't raised them from babies yourself.

With that said, a collection sale will be the way to go if you are set on breedable adults although these contain their own potential problems to look out for.