When Did Science Fantasy Digests Become Rare Collectibles?

After 25 years of buying vintage digest magazines on eBay, I’m rarely surprised. But I was left with my jaw hanging open last week when a set of 28 Science Fantasy magazines offered on eBay (above) blew past my $98 maximum bid, and ended up selling 9 minutes later for $1,580.
That’s not normal. I bought a set of 39 issues its sister magazine New Worlds just a few days earlier for $70, not even $2 per issue. Copies of Science Fantasy do tend to command higher prices, but not that much higher. So what’s going on?
It didn’t take long to figure out. Turns out the lot happened to include the June 1961 issue, containing Michael Moorcock’s “The Dreaming City,” the first appearance of Elric of Melniboné.