While he's mostly known for his cantilevered Bauhaus chair, Dutch designer Mart Stam sometimes scores another mention--at least in design auction catalogues. At least one product resulting from a "toy design seminar" he held in Dresden--the Schaukelwagen by Hans Brockhage and Erwin Andra--has made it into the history books.
Dated to 1950, the Schaukelwagen is a rocking car made out of bent beech and birch ply. Turn it one way, it's a car, sort of a Playsam-like roadster with the body panels removed. but flip it over, and it's a rocking chair, too.
Compared to Creative Playthings' Hobby Horse, it's a bit insider baseball, but still, the rocking chair's a pretty remarkable piece of modernist children's design. And at auctions over the last year, it's sold for EUR700-1250, with a tubular steel prototype going for EUR1400. [One also sold in 2004 for just $US325, though.]
With numbers like that, the rocking car that just went up on eBay with an opening bid of $400 [and $70 shipping to the US from Holland] doesn't seem too far-fetched. Good luck.
BROCKHAGE plywood rocking chair / car 1950ies Germany [ebay via dt reader/designshark andy]