In my research, I discovered that many churches, as well as other charitable groups and organizations, collect and use revenues from used stamps for projects, including missionary activity.
The history of stamps is, in itself, interesting. The Vatican was involved early in the whole process of mail delivery. The word "post," it appears, is derived from the "apostolic letters" sent out by popes. Postal services were delivered through messengers, followed by marking "paid" or sticky labels (later stamps) that could be affixed to a letter or parcel in the mid-1800s.
However the history of collecting used stamps for the missions is harder to come by. There is reference to collecting used stamps for the missions in Quebec as early as 1911.
Obviously, as the demand for stamps by hobby stamp collectors grew, so did the possibility of earning money for the missions through selling of stamps to collectors.
Sell to dealers
Charitable groups that collect stamps sell them to stamp dealers, either large or small. The dealers pay the charities for these stamps and the money is used by the organizations for their good works. Dealers then sell these stamps to individual stamp collectors or to smaller dealers who in turn sell them to individual collectors.
Commemorative stamps, limited edition stamps, old stamps, stamps with flaws such as upside down pictures are much more valuable than the everyday variety. Some organizations request only certain types of stamps or the more unusual stamps while others take all kinds.
It is important that stamps be in good condition with a margin of paper around them to protect the fringe that should not be damaged. Stamps that have an ink mark on them made by the post office are of no value. The same applies to metered postage.
Development and Peace, the Franciscans and probably many religious groups collect used stamps as fundraising for their mission work. Community League Re-use Fairs sometimes ask for stamps but I'm not sure how they use them.
Stamp dealers pay for stamps according to the good condition and uniqueness of the stamps. Some particular stamps can be worth hundreds of dollars. One group told me that they get $7 a pound for commemorative stamps. The revenue from large-scale sales can be considerable.
One group said that over about 15 years, it made close to $200,000. This is no small sum for something that otherwise would go into the garbage.
The need for stamps by philatelists won't diminish anytime soon. Therefore, I believe we should continue to collect used stamps especially now that email has decreased the number of stamps in circulation, making those that are still being used more valuable.
Some churches have stamp-collecting boxes. Ask your church about putting one out and begin collecting used stamps for these good causes. I can imagine that preparing stamps for sale must be a long and tedious process.
But for those who can find time to give, it is certainly a worthwhile and meritorious task to help the good work of charitable organizations.