What do I look for in a used sub? Do all subs bottom out?

[PlasmaMan]

I have seen several subs on Craigslist. BIC America is a good budget sub from what all I read. F12, V1220. But I refuse to buy anymore subs without hearing them. I have been burned before.

When I go to hear the sub, when should I hear the flapping noise? Never? Do they all do that when the bass hz is super low (for a given sub)?

If I put in War of the Worlds, should the sub start flapping, buzzing, whatever, if I turn the sound on the receiver up?

Thanks.