Looks just like a regular chipper on a wagon.
I searched Yahoo images and a few came up for sale. Not sure what model numbers they were tho.
Try posting it on your local CL and see what happens. Start out higher so you can be haggled. Know your lowest price and stick to it.
To tell the approximate age, find the Model/Type/Code on the engine.
xxxxxx xxxx-xx yyxxxxxxx
yy should be the year the engine was produced.