Build a Price Comparison Tool in 15 Minutes with the Marketplace Price API

Ever tried to find the best price for something across multiple marketplaces? You'd need to open OfferUp, Poshmark, Reverb, TCGPlayer — each with different search interfaces and no way to compare side by side.

The Marketplace Price API solves this with a single unified /search endpoint that queries multiple marketplaces simultaneously and returns normalized results.

In this tutorial, we'll build a CLI price comparison tool in under 15 minutes.

What We're Building

A Node.js script that:

• Takes a search query as input

• Searches across 4 marketplaces in parallel

• Shows a sorted comparison table with the best prices

Prerequisites

• Node.js 18+

• A free RapidAPI account (sign up here)

Step 1: Get Your API Key

Subscribe to the Marketplace Price Tracker API on RapidAPI. The free tier gives you 500 requests/month — plenty for this tutorial.

Copy your X-RapidAPI-Key from the API dashboard.

Step 2: Set Up the Project

mkdir price-compare && cd price-compare npm init -y

mkdir price-compare && cd price-compare npm init -y

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Create a .env file:

RAPIDAPI_KEY=your_key_here

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Step 3: Build the Comparison Tool

Create compare.mjs:

const API_KEY = process.env.RAPIDAPI_KEY; const BASE = "https://marketplace-price-tracker.p.rapidapi.com"; const HEADERS = {  "X-RapidAPI-Key": API_KEY,  "X-RapidAPI-Host": "marketplace-price-tracker.p.rapidapi.com", };

const API_KEY = process.env.RAPIDAPI_KEY; const BASE = "https://marketplace-price-tracker.p.rapidapi.com"; const HEADERS = {  "X-RapidAPI-Key": API_KEY,  "X-RapidAPI-Host": "marketplace-price-tracker.p.rapidapi.com", };

const query = process.argv[2]; if (!query) { console.error("Usage: node compare.mjs "); process.exit(1); }

console.log(\nSearching for "${query}" across 4 marketplaces...\n);

// Search all marketplaces in parallel const endpoints = [ { name: "Reverb", path: /reverb/search?query=${encodeURIComponent(query)}&limit=5 }, { name: "TCGPlayer", path: /tcg/search?query=${encodeURIComponent(query)}&limit=5 }, { name: "OfferUp", path: /offerup/search?query=${encodeURIComponent(query)}&limit=5 }, { name: "Poshmark", path: /poshmark/search?query=${encodeURIComponent(query)}&limit=5 }, ];

const results = await Promise.allSettled( endpoints.map(async ({ name, path }) => { const res = await fetch(${BASE}${path}, { headers: HEADERS }); if (!res.ok) return { name, items: [] }; const data = await res.json(); const items = (data.results || data.listings || []).map((item) => ({ marketplace: name, title: item.title || item.productName || item.name, price: parseFloat(item.price || item.marketPrice || item.lowestPrice || 0), condition: item.condition || "N/A", url: item.url || "#", })); return { name, items }; }) );

// Flatten and sort by price const allItems = results .filter((r) => r.status === "fulfilled") .flatMap((r) => r.value.items) .filter((item) => item.price > 0) .sort((a, b) => a.price - b.price);

if (allItems.length === 0) { console.log("No results found. Try a different search term."); process.exit(0); }

// Display results console.log("Price".padEnd(10) + "Marketplace".padEnd(14) + "Condition".padEnd(12) + "Title"); console.log("-".repeat(80));

for (const item of allItems.slice(0, 20)) { const price = $${item.price.toFixed(2)}.padEnd(10); const mp = item.marketplace.padEnd(14); const cond = (item.condition || "N/A").slice(0, 10).padEnd(12); const title = item.title?.slice(0, 44) || "—"; console.log(${price}${mp}${cond}${title}); }

console.log(\nFound ${allItems.length} results across ${results.filter((r) => r.status === "fulfilled").length} marketplaces);`

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Step 4: Run It

RAPIDAPI_KEY=your_key_here node compare.mjs "fender stratocaster"

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Output:

Searching for "fender stratocaster" across 4 marketplaces...

Price Marketplace Condition Title

$149.99 OfferUp Good Fender Squier Stratocaster Electric Gui $225.00 Poshmark N/A Fender Player Stratocaster HSS Buttercr $349.00 Reverb Excellent Fender Player Stratocaster 2019 3-Color $425.00 Reverb Very Good Fender American Stratocaster Sunburst $599.99 Reverb Excellent Fender American Pro II Stratocaster ...

Found 18 results across 4 marketplaces`

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Available Endpoints

Endpoint Description

GET /reverb/search Reverb music gear listings

GET /reverb/price-history Historical price data for Reverb items

GET /tcg/search TCGPlayer card listings

GET /tcg/price TCGPlayer card price lookup

GET /offerup/search OfferUp local marketplace listings

GET /poshmark/search Poshmark fashion resale listings

Use Cases

• Resellers: Find arbitrage opportunities across marketplaces

• Collectors: Track TCG card prices, find the best deals on vintage gear

• Shoppers: Compare prices before buying

• Discord Bots: Power price-check commands with real-time data

• Price Tracking Apps: Build dashboards with historical pricing

What's Next?

• Add a simple web UI with React or Svelte

• Store historical prices in SQLite for trend analysis

• Set up a cron job to check prices daily

• Build a Discord bot with /price-check commands

The Marketplace Price Tracker API free tier includes 500 requests/month — enough to build and test any of these.