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battles / Analytics

Cometly vs SellerLegend

Cometly ($399/mo/mo, vibe code 5/10) vs SellerLegend ($50/mo/mo, vibe code 5/10). Cometly is the easier one to rebuild yourself — here is what you lose either way.

Cometly
NICHE

Analytics

$399/mo/mo

Vibe code5/10
Moat2/10
MVP
1 week
Full replacement
3-6 months, due to CAPI integration maintenance and browser identity resolution changes

easier to rebuild

get the build prompt →
SellerLegend
NICHE

Analytics

$50/mo/mo

Vibe code5/10
Moat2/10
MVP
2 weeks
Full replacement
4-6 months, primarily due to Amazon SP-API app approval friction and fee reconciliation logic
get the build prompt →

price gap / year

$4,188/mo

running both / year

$5,388/mo

our call

Start with Cometly — highest vibe code, weakest moat.

Cometly

Building a basic UTM recorder with a dashboard takes a weekend. Replicating multi-touch attribution, server-side CAPI event deduplication, and maintaining ad platform API connections against breaking changes across Meta, Google, and TikTok is a full-time engineering effort.

you can rebuild

  • UTM parameter capturing and first-party cookie tracking scripts
  • First-touch and last-touch attribution SQL query logic
  • Analytics dashboard displaying CAC, ROAS, and ROI per ad campaign
  • Shopify webhook receiver for sales conversion ingestion
  • Basic server-side event payload generation

what you lose

  • Turnkey client/server event deduplication across Meta, TikTok, and Google
  • Automated token refresh and API schema maintenance across ad platforms
  • Pre-packaged identity stitching across multi-device user journeys
  • Ad spend ingestion pipelines that handle API rate limits and currency conversions
  • No-code Shopify theme app integration

real moats

  • Maintained native integrations with rapidly evolving ad platform APIs
  • First-party identity graph infrastructure optimized for browser privacy limits
  • Turnkey server-side tracking bypassing Safari ITP and ad blockers without custom DNS configuration

open source escape hatches

SellerLegend

SellerLegend is a calculation UI sitting on top of Amazon's Selling Partner API (SP-API). Rebuilding the frontend tables and math in Next.js is easy, but obtaining production SP-API developer approval from Amazon and handling strict rate limits requires non-trivial setup.

you can rebuild

  • Product-level real-time profit and loss calculations
  • Manual and batch COGS (Cost of Goods Sold) management
  • Amazon Sponsored Products / PPC ad spend attribution
  • Amazon FBA inventory health and fee breakdown tracking
  • Customizable SKU and ASIN financial performance tables

what you lose

  • Pre-approved Amazon SP-API developer credentials
  • Out-of-the-box handling of Amazon SP-API rate limiting and backoff
  • Automatic settlement file parser for 50+ Amazon fee types across global marketplaces
  • Automated historical backfill of past settlement cycles
  • Pre-built multi-currency conversion logic for foreign Amazon marketplaces

real moats

  • Approved Amazon SP-API Developer status (strict audit and Data Protection Policy compliance)
  • Historical transaction data store spanning years of seller history
  • Battle-tested settlement parsing engine for edge-case fee codes

open source escape hatches

Questions people ask

Which is easier to rebuild with AI, Cometly or SellerLegend?

Cometly. It scores 5/10 on vibe code with a moat of 2/10, so an AI-assisted MVP takes about 1 week and a full replacement about 3-6 months, due to CAPI integration maintenance and browser identity resolution changes.

Which one costs less, Cometly or SellerLegend?

SellerLegend at $50/mo/mo for a typical mid-market store. The gap between the two is about $4,188/mo a year.

What do I lose if I replace Cometly?

Turnkey client/server event deduplication across Meta, TikTok, and Google Automated token refresh and API schema maintenance across ad platforms Pre-packaged identity stitching across multi-device user journeys

What do I lose if I replace SellerLegend?

Pre-approved Amazon SP-API developer credentials Out-of-the-box handling of Amazon SP-API rate limiting and backoff Automatic settlement file parser for 50+ Amazon fee types across global marketplaces

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