SellWhere.app VS Diffyn

SellWhere.app's answer

SellWhere is the only free tool that lets you enter a sale price once and instantly compare your net payout across 6 major resale platforms — eBay, Poshmark, Mercari, Vinted, Depop, and Etsy. Most fee calculators only handle one platform at a time.

Our eBay calculator goes deeper than any competitor, modeling 20+ item categories, 6 store subscription tiers, and 3 seller levels. Every calculator shows when fees were last verified, with direct links to official fee pages so you can trust the numbers.

Diffyn's answer:

Addresses workflow and change management on LLM prompts, provide teams with traceability and visualization of tests across multiple models, provide deeper understading into efficiency of these prompts.

Diffyn's answer:

Diffyn is the platform that specializes on both change management and multi-model analysis.

SellWhere.app's answer

• Astro — static site framework for fast, SEO-friendly pages
• React — interactive calculator components (Astro Islands architecture, so only calculators load JavaScript)
• Tailwind CSS v4 — utility-first styling
• Cloudflare Pages — global edge deployment for fast load times worldwide

Diffyn's answer:

React, Next.js, POSTGRESQL

SellWhere.app's answer

SellWhere serves two main groups:

• Casual sellers clearing out closets or decluttering, who want to know which platform gives them the best payout before listing
• Professional resellers who list across multiple platforms (eBay, Poshmark, Mercari, etc.) and need accurate, up-to-date fee comparisons to maximize their margins on every item

Diffyn's answer:

Professionals incorporating LLMs or AI tools in their workflow and wants to keep track of changes and test their prompts.

SellWhere.app's answer

SellWhere was born from a real frustration: resale platform fees are confusing, constantly changing, and scattered across different help pages. We wanted one place to see exactly what you'd take home on every major platform, with no guesswork.

Resale fees are also in a period of rapid change — Mercari dropped its seller fee to 0% in 2024, Depop eliminated buyer fees in mid-2024, and Vinted launched in the US in January 2026. Keeping up is hard. SellWhere was built to be the single source of truth for resellers navigating these shifts.

Diffyn's answer:

I started working on Diffyn when I notice that prompting has become an essential part of work across many industries. While there are version control platofrms like github, they are not designed for just prompt management are can be overkill such applications, it is also not integrated natively with various LLMs and relevant tools for users to validate ideas and visualise results properly.