First stamps or sets of stamps of specific countries
Surely a good idea. There are lots of stamps or sets of stamps that were issued as #1 in the country and are very valuable nowadays.
Têtê-Bêchê
Tête-bêche is a joined pair of stamps in which one item is upside down in realtion to the other. These may be produced accidentaly or intentionally. These pairs may be horizontal or vertical. Tête-bêche is a French word meaning „head to tail“.
Printing or other errors
Stamp is a printed piece of paper. Errors may occur during the printing proces. These errors are usually very rare and therefore the stamps containing these errors are valuable.
Colonial stamps and issues for foreigner Post offices including watermark and perforation faults, high denominations,…
High denominations of colonial stamps, postage stamps issued by foreign post offices, watermark errors etc. are usually very valuable. there are not many of them and it makes them scarce.
Stamps issued or overprinted by occupation authorities
These stamps were usually not in use for a long time and therefore not many of them survived.
Stamps with a “story” and good provenance
Story and provenance sells. A stamp which used to be in the collection of Barone de Rotschild will always be more valuable than the same item without this provenance.
Trial prints and plate flaws – the whole world until 1950
Not many of them survived and that is the reason why they grow in price from the very beginning.
Unissued stamps
Unissued means that they were printed but the issuance had been stopped shortly before the distribution was planned. Again, not many survived, as the most of them were officialy destroyed.
Stamps of countries with the long stamp collecting tradition
If there is a tradition, it is likely to carry on :-)
Propaganda Cards from WWI and WWII
Interesting items with historical background. Surely worth to collect and invest in. The prices are quite stable and appreciate in time step by step.