Those rifles are listed as new.

Hermann's Sporting Guns: Price List

MERKEL 140 A DOUBLE BARREL SIDE BY SIDE DANGEROUS GAME RIFLE, GREENER CROSS BOLT WITH DOUBLE UNDER BARREL LOCKING LUGS ,COCKING INDICATORS, SIDE CLIPS AND RECEIVER LEDGE, CASE HARDENED RECEIVER, IN CAL. 375 H&H,416 RIGBY, 470 NE. A$8999 US$5219 NEW

The higher-grade engraved model is more in line with the US prices you quoted, jscybercat:

MERKEL 140 A DOUBLE BARREL SIDE BY SIDE DANGEROUS GAME RIFLE, GREENER CROSS BOLT WITH DOUBLE UNDER BARREL LOCKING LUGS , COCKING INDICATORS, SIDE CLIPS AND RECEIVER LEDGE, FULLY HAND ENGRAVED, IN CAL. 375 H&H,416 RIGBY, 470 NE. A$10999 US$6495 NEW

Just from what I can gather from the web and this forum, double rifles both new and used are priced much more sensibly in Australia than the US. This is especially so for vintage Brit guns. In fact I better shut up about this, its been a well-kept secret for years!