MarginLog

The problem hit me when I was flipping on Poshmark and eBay on the side.

I'd sell something for $45 and feel good about it. Then I'd actually sit down and do the math — Poshmark's 20% cut, the $8 I paid to ship it, the $12 I paid at the thrift store — and I'd made $7. Sometimes less.

I was running a spreadsheet that I kept breaking. I had no visibility into which platform was actually profitable, which categories made me money, or what my real margins looked like over time.

The tools that existed were either built for serious e-commerce businesses (way more than I needed, expensive) or they only calculated fees for one platform at a time.

So I built MarginLog.

What it does:

• Logs every sale in ~30 seconds

• Calculates true net profit after platform fees, shipping, and cost of goods — for eBay, Poshmark, Mercari, Depop, Etsy, Facebook Marketplace, and Vinted

• Shows you profit by platform, category, and month

• Exports to CSV for tax season

• Tracks sourcing trips so you know if that Goodwill run was actually worth it

Where it is right now:
Early beta. Free for the first 15 users — no card, no time limit. I wanted real people using it before I thought about pricing.

I also just shipped free standalone calculators for each platform (no signup needed) to drive SEO traffic. And a compare page that shows your profit across all 7 platforms side by side from a single input — useful for deciding where to list.

What I've learned so far:
Resellers don't think of themselves as running a business. They think of themselves as people who like to flip stuff. The product language that resonates isn't "SaaS analytics platform" — it's "finally know what you actually made."

Would love feedback from anyone who resells, or anyone who's tried to build for a non-technical audience. What am I missing?

Link: marginlog.vercel.app