The first stamp ever issued continues to be one of the most interesting. Over 68 million Penny Blacks were issued and a higher percentage of these survived than with later stamps as most of them were used on folded letters (where the letter portion is written and then folded under so that the paper forms its own envelope). Penny Blacks are not scarce but they have always been popular and they continue to fascinate. They were printed on twelve plates and each stamp on each plate was given a different check letter number so that after removal from the sheet postal officials could tell where on the sheet the stamp came from. The stamps were printed in sheets of 240 stamps (twelve rows by twenty rows) so that specialists who try to obtain all of the check letters from all of the plates need 2640 stamps for completion.
Prices for nice quality Penny Blacks have been rising quickly over the last year or so and the reason is Chinese demand. The Chinese have become avid collectors and are maturing in their philatelic tastes beyond their home markets. Stanley Gibbons reports enormous trade orders from China for Penny Blacks http://www.youtube.com/watch?v=Zx54IUBpOtg.
The issue for Penny Black prices (and really for almost any older stamp) is that quantities in dealer hands typically are small. When certain stamps are wanted for promotions, prices can rise very quickly. Penny Blacks have doubled in price in the last eighteen months and look to go higher.