Air rifle Piefer Bayard from Belgium Herstal
Well Vincent, it's quite a nice looking rifle and seems well made too, so maybe I'd start at 150 euros or so, and choose a lower starting price if it doesn't sell. But who knows, perhaps there is a vibrant market for these in Belgium? At least with an auction you'll find out.
I'm not aware of an auction website for airguns in Belgium but I do know that in Germany there is eGun.de.