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Most collectors would consider it a defacement and reduction in value.
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When they retire? Country antique dealers...
See link to - American Stamp Dealers Association
A trade mark, manufacturer's mark or factory mark.
The Post Office. Stamp Collectors. Stamp Dealers.
American Stamp Dealers Association
There are a variety of auction houses and stamp dealers. Check your local yellow pages for dealers in your area. I would recommend obtaining a copy of some of the philatelic journals, Stamp Weekly or Lynn's Stamp News. There will be many ads offering to appraise collections. The American Philatelic Association can provide names and addresses of dealers also.
The One Penny Black is the first stamp created. Most dealers will have them for sale. You can also find them on auction sites as well.
Any dealer or stamp store would likely have a good selection of stamps from Africa.You can go to the web page for Linn's stamp News , www.linns.com/ or go to eBay and look at the auction offerings to get started. You might also Google 'stamp dealers". There are hundred of dealers who have web pages.
Your local phone book would be one source. The other key place would be the American Stamp Dealers Association web site has a dealers listing that can help you locate a dealer.
You can buy non-judicial stamp paper from stamp paper dealers in Mumbai. The non-judicial paper is only valid for 6 months after its purchase.
Most dealers are not going to be interested in buying a single stamp. You might try selling it on an auction site, but the postage cost will be more than the value of the stamp.