Collecting postmarks

Collecting postmarks is an odd little sub-hobby of postal history. In a way, it ignores almost all the interesting bits about postal history (routes, rates, mail types, etc.) and focuses on the markings that are only of indirect interest.
That said, it's a lot of fun! I'd argue that it's often the most natural expansion of a stamp collection, to move into collecting the postmarks on the stamps. Since postmarks on loose stamps can be frustrating, it's often easier and more rewarding to collect postmarks on complete items of mail.
Oversimplifying recklessly, I think you can collect postmarks in four ways: by location, period, type or usage. Location and period are fairly self-explanatory while "type" can be as simple as "oval" or as complicated as "locally produced nonstandard datestamps of the late Imperial Russian period" (and if you collect the latter then good luck - they're fascinating and very, very scarce). "Usage" is the least popular category and includes perfectly boring-looking postmarks used by seasonal post offices. It's often an indirect way to define "type".

As I may have mentioned once or twice, I collect postmarks from the Crimea, pre-stamp to 1945. The late Imperial (1903-1917) and early Soviet (1923-1939) periods are by far the most interesting to me. In the late Imperial period, three classes of Crimean postmarks are a particular headache to find.

Railways post offices include the two Zh.D.P.O.-class offices of Sevastopol' (common) and Kerch' (rare); three stantsiyas which were converted to ordinary P.O.s fairly soon after their opening (Nikita, Sarabuz and Sem' Kolodezei - I've only seen the latter); and regular stantsiya offices. I had never seen a Crimean stantsiya postmark until fairly recently, when a Soviet late usage of the Imperial postmark of Akmanai found its way into my collection.

Sem' Kolodezei as a P.O.

The second class of "difficult" postmarks are the seasonals. Livadiya is the oldest and the most common.Six new seasonal offices were opened in 1915: Isary, Koktebel', Moinak, Otuzskiya Dachi, Sara-Kurort and Chokrak, of which Sara-Kurort was converted to a regular P.O. in 1916. The last to be added was Sudakskiya Dachi in 1917. Sadly, Evpatoriiskiya Dachi, despite its summer-y name, was not a seasonal office.

Otuzskiya Dachi Seasonal office

The final class of problem cases are the "Volostnoe Pravlenie" offices. There were 9 of them, of which 6 were converted to P.O.s in 1916 I have yet to see a postmark of any of them...