At/near retirement at MSRP (+ GWP), or 20–35% off clearance. Everything else is noise.
There are only two valid 'buy' conditions in the doctrine. First: a set on the official LEGO retirement list (or showing strong retirement signals — limited LEGO.com availability, no restocks) bought at MSRP, ideally during a Gift-With-Purchase promotion that adds free flippable inventory. Second: clearance at a 20-35% discount off MSRP, which compresses the cost basis enough to maintain the 250% net target even on weaker performers.
Most LEGO investors lose money by buying outside these windows. They buy hyped new releases at MSRP and watch them get discounted six months later. They buy at 10% off thinking they're getting a deal — but 10% doesn't move the math enough to matter. The two-signal rule is the discipline that keeps your portfolio's average cost basis low enough for the strategy to work.