Be aware that you should be "fitted" for a N95 mask. The hospitals use a sweet tasting chemical. You wear a bag over your head while you have the mask on underneath. You read from a script (little rainbows and their pot of gold <img src="/images/graemlins/smile.gif" alt="" /> designed to make you work your jaw. If you can't taste the chemical, you have a good seal. You could probably improvise a test, perhaps masks boxed for home use have instructions.

"Military" style gas masks have very specific purposes. Unless you carry it with you, expecting to be tear gassed or traveling through areas hit with chemical munitions, they won't really help you. I have one merely for S&Gs, no real expectation to ever have to use it. Unless there is a scheduled artillery barrage on my house, I would either A) die from exposure before I would know to don my mask, or B) have time to seal the house and therefore not need the mask. It would take a hell of a lot of chemical to douse an area large enough that you couldn't avoid it.

I would be very wary of buying any mil-surp masks off of ebay. Stick to NIB masks designed for the civilian market if you really want one.