That $15,000 Prince album, Pink Floyd's eternal popularity and other findings from the Discogs annual report

Devoted to good taste (mostly) and the vanishing pleasures of physical music, Discogs, which launched in 2000, was built by and for music nerds. It's an awesomely comprehensive and addictively appealing rabbit hole of completist geekery. "Without sounding snobby, we're more that connoisseur-level collector," said Ron Rich, Discogs's director of marketing.

Discogs offers a user-driven database, meticulously kept, which seeks to catalogue discographical information for every record ever made (there are more than 8.2 million entries). It's also a global, eBay-like marketplace where those records are bought and sold, complete with user groups and forums, and an app.

Discogs's newly released 2016 sales report, provided exclusively to The Washington Post, serves as a musical State of the State address for the site. It clocks trends in physical music sales and details the current obsessions of record collectors, who are strange creatures willing to spend $4,329 on a 1971 LP from British rock band Leaf Hound but also rational actors who appreciate regular people things, like "Thriller" and Pink Floyd.

Things we learned on a deep dive (all figures are Discogs's own):

A promo copy of Prince's "Black Album" became the most expensive title ever sold on the site. A double 12-inch version of the album, which Prince ordered recalled a week before it was to be released in 1987, fetched $15,000 shortly after his death last April. "The Black Album" has long been a holy grail for collectors. Most copies of the album had been destroyed; this one, apparently intended for a DJ, eluded capture. In second place: a first pressing of "David Bowie" (later reissued as "Space Oddity") sold for $6,826.

The artists on Discogs's "Most Collected" chart are mostly white, male and dead. "Most Collected" is Discogs's version of Billboard's catalogue albums chart, and it's weighted toward the usual classic rock suspects: the Beatles, the Eagles, Led Zeppelin. One-tenth of the chart belongs to Pink Floyd, including the top two slots ("The Dark Side of the Moon" and "Wish You Were Here," respectively). The only contemporary act in the top 50 was Radiohead (at No. 19 with "A Moon Shaped Pool"); the only women are the distaff half of Fleetwood Mac.

Discogs's collectors love brass and military music. Really, unaccountably love it. Discogs does its core business in electronic and rock, but military music is a rapidly growing genre on the site, with collections up 55.83 percent in the past year alone. One possible reason: Military music aficionados, starved of online options, set up base camp at Discogs, filling in database information and selling and collecting music, and serving as a beacon to other, like-minded collectors.

For every predictable boomer favorite on the Discogs charts, there is another artist of baffling randomness. A hard-to-find vinyl edition of obscure psych-rock band Phafner's 1971 release "Overdrive" sold for $5,500 last year. It was 2016's most expensive release unrelated to Bowie or Prince. The priciest seller of 2015 was a rare copy of the 1989 hardcore release "Chung King Can Suck It," by Judge, which surprised everyone by selling for $6,048. Judge was an NYC straight edge band; Chung King was the recording studio with which it was feuding, for reasons too complicated to go into here. Very few copies of the album were made.

Discogs is powered by rarity and randomness. Nostalgia-driven record collectors are a huge section of the market, but their buying patterns are unpredictable, because everyone is nostalgic for different things. Other serious collectors are easier to figure, because they are reliably drawn to mishaps and scarcity - the DJ copy with Scotch tape covering a misprinted label, the elusive first pressing. Without artists misspelling their own album titles, dying unexpectedly, or releasing limited-pressing 7-inches on whimsically colored vinyl, record collectors would have a lot less to collect.

Collectors still love cassettes. Marketplace cassette sales grew almost 40 percent this year, though they're still far outstripped by sales of vinyl, which account for 6,691,144 of the site's 8,311,646 total sales. In August, "The Versace Experience - Prelude 2 Gold," a Prince promo tape distributed only at a 1995 Versace fashion show, sold for $4,087, the highest cassette sale price the site has ever recorded.

Once a collectible is sold, it's anybody's guess what happens to it. Will anyone ever actually play "The Versace Experience"? Or will it wind up in a temperature-controlled vault somewhere, awaiting resale during the 2036 Prince revival? Cassettes can deteriorate if played too often or improperly stored, potentially limiting their value as an investment. Vinyl can also degrade with too much wear, but, as marketing director Rich notes, many vinyl connoisseurs don't even have turntables. They view vinyl as a tactile experience; liner notes are to be pored over, LP covers displayed.