Hi Sorcecode, depending on which web site you saw those numbers on, they are usually stamp catalog numbers. In the USA and Canada we generally use Scott's Standard Catalog, or Scott, which is numbered from the first printed stamp issued for postal use, #1 being the first stamp issued, etc.
So, if on
ebay US (dot com) then the #'s are usually Scott (if the seller is in Great Britain (United Kingdom) the numbers are from the Stanley Gibbon's Catalog or SG). Other countries and regions use different catalogs and numbering systems.
Check on here before buying a used catalog as certain years are better than others.
I do not have a US catalog so I will let others describe the difference between the chaeper stamp and the way expensive one.
Doug