They can make concrete as smooth as glass.
The dimensions of a basketball court are 50' by 94'.
Even if all you had was a 4" slab, that is probably
going to cost you $5000 just for the concrete and not
including the cost of the forms, the remesh, the
reinforcement bars, the bull floats, and the powered
concrete paddles. My guess is that all in all, it
will approach $10,0000. On top of top that, if there
are no expansion joints, that size of surface will
expand and contract in the weather and there will be
cracks all over it. Any use you want for it will
only last a short time.
I don't think you have thought this through. You
either have a lot of money to waste or this is just a
pipe dream. Sorry, but I do not think it is
practical. That is why all roller blade rinks I know
of are inside with wood floors. You can make outside
basket ball courts, but there are lots of seams and
usually they are half courts unless you have school,
city, or county funds.
If you are willing to just settle for a hockey rink,
this can be done for about $100 - $500. In the link
below, a 50 by 25 skating rink is constructed for under
$200. Also there is a link on that page to a slightly
larger rink built with PVC pipe for $300. Just get a
level area with side barriers and fill with water.