“there's a a really interesting case study about Airbnb where they also track uh retention ured even though there's no subscription because they estimate that people book One Vacation per year uh with Airbnb and so their retention is basically one one booking per year”

For non-subscription marketplaces, define retention against expected booking frequency

Just because you're not subscription doesn't mean you stop tracking retention — you redefine it. Estimate your category's natural booking cadence (Airbnb: ~1 trip/year, MentorCruise: per-session or per-package), then measure whether buyers come back at that cadence. Without this number you can't tell whether marketplaces are leaking buyers or just operating at their natural frequency.