In the aftermath of a battle there would have been a lot of armour and weaponry left lying around, either on the dead or left by fleeing soldiers. What happened to this? Was it left where it lay, did the victorious side collect it all, did looters grab it? What was stopping an entrepreneur from hanging around battle sites and collecting all the fallen stuff and re selling it?
A lot of this would depend on the cultural practices of particular peoples. It would help if you could specify what period and region you're interested in!