I can see that, would prefer to see movement, taper and nebari. Consider a tree when these three are present or developable!
In my opinion this tree is not a good choice at any price! Unless you wish to learn what not to buy!
The thickness and age appears as a straight short section. The multi branch above the straight section does not lend itself to a natural birch styling and as you pointed out yourself the roots are an unknown.
When purchasing a tree for bonsai try to select the positives that are the most difficult to change or develop.
The three biggies in my mind are nebari, trunk and branch placement or potential.
It does take time initially but it will save years of wasted effort if you can select better starter material before beginning.
There are many great bonsai nurseries in California to select starter material from that do not cost an arm and a leg!
Save a bit more and buy better quality it is available.
Within a short drive of your location, here are two I am familiar with.
Grove Way Nursery ( go to the back on the left for pre-bonsai choices )
Lone Pine Nursery ( larger selection, bigger location)
I am sure other members of the forum can add many more. Not too mention the many club auction in the Bay Area.