Lots of LP12 for sale...because there were SO many LP12’s made over the years, because they’ve been good and popular for a long time. They’re still in demand today. They have an extensive upgrade path. And more importantly, because they hold their value so well that even satisfied owners can be tempted to sell. People here often read the wrong direction into "always see X for sale" or "never see Y for sale" (the latter usually means it’s faded into oblivion, sometimes for good reason - why bother buying an ad for an item nobody will buy?).
Not an LP12 owner...but their place in hifi analog history has to be respected.