Collecting stamps for your charity is an activity that can involve the whole community.
Charity Kiloware are keywords that will help you search online for information and contact details.
ebay is one auction outlet that can help you sell - search for
Kiloware Stamps
Most buyers of collected stamps seem to prefer:
Unpicked stamps - don't remove them from their paper.
Unsorted stamps - sell on what you collect/have donated without going through them and removing anything !
5mm Border - trim the paper around the stamp
If possible collect in 2 boxes - GB and International
Some buyers will provide free post envelopes although the total cost per kilo may be less than those who pay their own P&P
If your group isn't organising any stamp collections and you still want to save & contribute, many Charities will gladly take reciept of them:
RNIB Stamp collection Scheme
Haworth Cat rescue collect.
RSPB - and
Hearing Dogs - (scroll down to the bottom of the page)
Have the same stamp dealer address so I would guess that the charity of your choice needs to be included with your parcel.
I have only looked into this briefly so would value others experiences and views ..