I buy for cash (preferably) or swap for things I lack in my collections. I have two collections - one is Danish/Norwegian and Norwegian military arms and blades, the other one is of rifles designed by Hans Larsen or his son August.
For the military collection, condition is very important. I have a number of Danish/Norwegian blades and muskets/rifles as doublets, including things like the M1711/1841/51 'skirunners rifle' with the original stock, a load of dragonpalasker (heavy cavalry swords with brass baskets) etc to swap, but prefer cash. Here you find Norwegian military arms:
http://norskevaapen.no/
Hans Larsen rifles are usually marked Hans Larsen Drammen, Larsen Liege, Larsen Rifle company etc. Here I can swap with Larsen doublets or military doublets or cash. Here mechanism is more important than condition. Here you find some Hans Larsen arms:
http://norskevaapen.no/?p=513