Stamps validity?

If I recall, sometime around the Civil War all prior issued US stamps had been demonetized. All subsequent stamps should be still valid for postal use but some have restrictions such as bulk mail.

Generally speaking a stamp is valid until the postal authority declares it invalidated!  ...It varies from country to country.

In the UK we can use any GB stamps from 1972 to date, i.e. in decimal currency. 

US stamps much older are still valid, all those 3c stamps from the 1930s can still be used. Probably many older, but there will be many oldies where the catalogue value is greater than the postage value so only an idiot would use classic stamps in the mail.

I imagine "BULK MAIL" is where a charity or similar sends out a blitz of hundreds or thousands of mail shot envelopes and has a licence from USPS to use Bulk MAIL rate stamps, much cheaper than regular postage.

Nothing to do with BULKY PACKAGES ! 

I see that you are now living in France. France issues stamps in EURO,  but stamps marked in FRANCS are still valid for postage. There is an official exchange rate.

I have even see people get away with using old Francs which were devalued to 1/100th in the 1960s, but that is not correct legal usage.

As vikingeck noted, bulk mail in the US refers to the total mail volume for a particular mailing, rather than the physical size of each piece of mail.

It is available to for-profit organizations as well. The non-profit organizations get an even greater discount. There is a fee, and the permit is only valid for use at the specific post office the permit is filed when doing actual bulk mailing (for accounting purposes). A lot of other requirements as well, but won't get into that.

In the US, collectors can also obtain a no-cost permit, which allows them to use bulk mail and pre-canceled stamps on first class mail. In practice, many collectors don't bother dropping off the mail at the post office where the permit is on file. As long as you have at least one tagged stamp, and mark the mail properly as "FIRST CLASS", it will go through. It's been my experience that dropping of the mail at the PO counter tends to bring more grief than it's worth -- especially when there are new counter clerks or a new station manager! So I admit, I'm one of those who simply tosses it in the mail to save myself the 30 minutes of explaining/showing the rules to the postal clerks/manager!

The "collector" permit, you only have to get once, and you are not assigned a number, unlike the fee-based annual permits that "real" bulk-mailers must have to get the discount.

Regarding using old unused postage. I was spoiled by the lack of US rules regarding demonetization (other than the Civil War demonetization and a few minor category demonetization rules). One trip to Asia 30 years ago, I was shocked to find out that issues even a few years old were no longer valid. I was told the post office would post notices that regularly demonetized certain stamps (mainly commemoratives) after only a few years! Don't know if that's still the case.

Interesting. I wondering how do the poste keep track of real issued stamps and make up one. The US issued a lot of stamps per year no?

Well, thank you for your very contructed response! I was just wondering I wonder why on eBay, older stamps are being sold much cheaper than their face value?