Spencer Pratt Sold Pics of Teen Mary-Kate Olsen Partying With Henry Winkler's Son

Spencer Pratt started working on his reputation as a villain long before he became one on reality TV ... he's admitted he once sold snaps of a teenage Mary-Kate Olsen partying with her then-boyfriend, Max Winkler, for a whopping $50,000!

Spencer spilled the deets in his brand new memoir "The Guy You Loved to Hate: Confessions from a Reality TV Villain" ... and he's saying he doesn't regret it a single bit.

Here's the deal ... he writes his pal Max -- the son of actor Henry Winkler -- had just gone through a split with Mary-Kate ... but still had a "shrine" of photos with her in his room. He describes the images as "young love documented in European hotels, Hollywood parties, stolen moments."

Well, Spencer thought he'd do his friend a solid by removing the party pics from his room ... which Max apparently did not protest. But, instead of taking the photos to the trash ... Spencer sold them to a photo agency, which then used them to spin a story about Mary-Kate having a bad-girl party phase.

Spencer writes in his memoir, he got $50K for the snaps, and used that to finance a film he wanted to make ... though it never saw the light of day.

And even to this day, Spencer says he feels no remorse. He explains ... "When you really think about it, it was a win-win. Mary-Kate got her rebel rebrand, Max got closure."