David,
Playing in this field, a few gavel auctions need to be monitored, even if buying nothing from them. They are:
GMA:
http://www.gregmartinauctions.com (this is the one with quite a few Lugers listed now)
RIA:
http://www.rockislandauction.com
JJA:
http://www.juliaauctions.com
JCD:
http://www.jcdevine.com
LJA:
http://littlejohnauctionservice.com
B&B:
http://www.bonhams.com
But nowadays, two proxies make life easier. Not only well known U.S. auctioneers go there, even European Hermann Historica goes there:
http://www.proxibid.com
http://www.artfact.com
Hope this helps. Registration is free.