Hi 'Nando,
Though this is not my field of expertise, from all I have heard from the international market, I cannot but comply with your experience!

German auction houses have tended for years not to accept 'prehistoric' items unless they have a documented provenance of at least before 1960. As far as I have heard both Stone and Bronze Age artifacts are being faked at large, mainly in Eastern European countries. Those guys even use fragments of historic bronzes to cast desired items - they are said to not even be told apart even by by refined scientific methods (apart from the fact that such an investigation would be costly far beyond the item's actual value) ...
Neolithic stone axe blades like yours without safe provenance would currently start at an estimate of ca. 20 to 30 euro (15 to 33 USD). Most of them remain unsold ...
Best,
Michl