First paying customer (and they weren't who I built for)

Builder Brief just got its first paying customer, 30+ days after launch.

Context: Builder Brief scans Reddit, HN, Indie Hackers, and founder newsletters every few hours and turns recurring complaints into structured problem briefs. Problem statement, validation evidence scored 1-5, competitors with pricing, source links. Free to browse, first brief free, $4.99 per download after. Built solo, after my last product (FollowUp) flopped at $18 in revenue across 4 months.

The interesting part isn't the milestone, it's that the buyer broke my assumptions.

I built Builder Brief for solo indie founders making $5 validation calls. The first person to pay is a researcher in the Netherlands who bought a marketing/growth brief, an LLM SEO audit tool. Not my ICP, not my market, not my content category.

So I pulled the first 40 users into a spreadsheet. Marketing & Growth briefs over-index 2.31x in saves vs their share of the catalog. Developer Tools, my single biggest category, under-indexes at 0.52x. I built a catalog heavy on the exact category my real audience cares about least.

The lesson connecting this to FollowUp: last time I validated the idea but not the artifact. This time I validated the artifact but assumed the audience. Same root mistake, different layer, I committed to an assumption before the data was in.

What I'm doing about it: not repositioning on 45 users (too thin), but broadening the next signal source toward where the audience actually is, and recalibrating at 100 and 250 users before touching positioning.

If you've had a first paying customer who wasn't who you built for, curious how you handled it, reposition toward them, or hold your focus and let them self-serve?