Hey Mike,
Preventive maintenance is always a good idea..[to prevent a breakdown] However, major failure of a tool [burnt up] is usually caused by incorrect voltage [extension cords,operating on the same circuit with other equipment ] heavy or improper use, too light of a tool for the work...not wearing corrective shoes..etc..
Replacing the brush's in this case probably wouldn't have changed the outcome..
it might be better to repair yourself instead of the ebay option.. unless they can guarantee the quality of the saw..:)
[except if your tool has more problems then just the armature..then i would go for the used one..]:p
just my two cents..
cmoorejack