Need a good chair for trading

Define old.
Don't get me wrong the swopper is a great chair and has its uses, but I doubt its of good use for continuous 8h+ workspace use.

The reasoning is very simple: While getting the back muscles to work does indeed promote back health because the muscles are forced to work, grow to some extend, and be stronger does cure any problems related to weak back muscles...
Its not intended by nature that you "stand" all day long, meaning having no back support. This may, over time, even lead to increased wear and tear of the spinal discs in the lower spine, and overstress the same muscles it intended to strengthen.

So it may be great as a second chair that you can rotate it with... but its nothing for 8hours of work. and btw, there is no 8hour work certification for those chairs, at least I found none when browsing a few minutes on their website.

just saying...

Too bad the Topstar products are not available in the USA. But you might look for chairs with similar features...

This is a very good chair

http://www.swingchair.com/

To describe what is preferred and natural -- A chair that lets the back flow and move at its own speed and direction.

Removing the chair base of a common office chair allows this to happen on the horizontal plane, given a sturdy platform to position the post on; metal rails help with any initial or occasional extra challenge to balance or to positioning the chair.
For the vertical plane, I like the noted chair's suspension spring to supplement the seat cushion and positioning (and some air cushioning) from a pneumatic-lift mechanism (in lots of office chairs).