In Ancient Greece, there were three ways to deal with prisoners: ransom them, sell them as slaves, or kill them. Despite the insistence of some modern scholars, the Greeks do not appear to have had a particular preference for any of these options. They sold captured enemies into slavery if it suited them, or butchered them if it suited them. Ransoming, though relatively rare, also occasionally happened. All three acts could be presented as correct, legitimate and desirable.

There is no explicit evidence as to what happened with the equipment of captives, but, given the options for the treatment of the men themselves, it seems extremely likely that prisoners lucky enough to be ransomed would at least be deprived of their gear. Equipment taken in battle was always a prestigious possession, and part of the loot was usually dedicated to one god or another. No commander in his right mind would take his enemy's weapons and armour only to give it back.

The best piece on this is Hans van Wees' 'Defeat and Destruction: The Ethics of Ancient Greek Warfare', in S. Tausend (ed.), Böser Krieg (Bayreuth 2011). This chapter is pretty hard to get, though; more easy to find is John Dayton's The Athletes of War (Toronto 2005), which has a pretty good section on the treatment of prisoners.