any tears in the sub surround should let you know that its been abused.
ask whoever your buying it from what else they were running in their system. If they were running a 300 watt RMS sub and a 1000w RMS amp, they probably beat the crap out of that sub...
test out the equipment, ask to hear it before you buy it.
and never buy a capacitor. because they're useless unless your into competitive audio, in which case you don't need to buy anything, because all of it will be given to you by your sponsors.
Feel free to call anyone who has a capacitor in their system an idiot. Ask them if they have a DMM (digital multimeter). if they do, they likely used it to set their amp's gains, which is good. Anyone who doesn't use a DMM to set their gains is likely to have sent a clipped signal to their equipment, and probably abused it as well.
I'd recommend looking in the classifieds section here and finding things you want.