How to build a CS2 skin arbitrage bot using the Skinstrack API

buy_price       = 11.92         # Buff163 ask
sell_price      = 13.10         # Skinport ask we list under
skinport_fee    = 0.12          # 12% seller fee

proceeds        = sell_price * (1 - skinport_fee)   # 11.528
net_profit      = proceeds - buy_price              # -0.392
net_roi         = net_profit / buy_price            # -3.3%

# Skip. Try sell on Buff163 instead (2.5% fee):
proceeds        = 12.50 * (1 - 0.025)               # 12.187
net_profit      = proceeds - 11.92                  # +0.267
net_roi         = +2.2%                             # still thin

curl -H "X-API-KEY: $SKINSTRACK_KEY" \
  "https://api.skinstrack.com/v1/paid/items?providers=csfloat,buff,skinport,waxpeer,dmarket&avg=true"

// Marketplace seller fees (sample, verify on each market).
const SELLER_FEE = {
  steam:    0.13,   // ~13% combined Steam fee
  buff:     0.025,  // 2.5%
  csfloat:  0.02,   // 2%
  skinport: 0.12,   // 12%
  waxpeer:  0.05,   // 5%
  dmarket:  0.05,   // 5%
};

function findArb(item, minRoi = 0.10, minLiquidity = 40) {
  if (item.liquidity < minLiquidity) return null;

  const buy = item.prices.reduce((a, b) => (a.price < b.price ? a : b));
  const sell = item.prices.reduce((a, b) => (a.price > b.price ? a : b));

  const sellerFee = SELLER_FEE[sell.provider] ?? 0.10;
  const net = sell.price * (1 - sellerFee) - buy.price;
  const roi = net / buy.price;

  if (roi < minRoi) return null;
  return {
    name: item.market_hash_name,
    buy: { provider: buy.provider, price: buy.price, count: buy.count },
    sell: { provider: sell.provider, price: sell.price, count: sell.count },
    roi,
    liquidity: item.liquidity,
  };
}

async function postToDiscord(webhook, opp) {
  const body = {
    content:
      '**' + opp.name + '**\n' +
      'Buy on ' + opp.buy.provider + ' @ $' + opp.buy.price.toFixed(2) + '\n' +
      'Sell on ' + opp.sell.provider + ' @ $' + opp.sell.price.toFixed(2) + '\n' +
      'Net ROI: ' + (opp.roi * 100).toFixed(1) + '%  (liquidity ' + opp.liquidity + ')',
  };
  await fetch(webhook, {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify(body),
  });
}

// minimal-arb-bot.js  -  run: node minimal-arb-bot.js
import 'dotenv/config';

const KEY = process.env.SKINSTRACK_KEY;
const WEBHOOK = process.env.DISCORD_WEBHOOK;
const PROVIDERS = ['csfloat', 'buff', 'skinport', 'waxpeer', 'dmarket'];
const MIN_ROI = 0.10;
const MIN_LIQUIDITY = 40;
const POLL_MS = 5 * 60 * 1000;

const SELLER_FEE = {
  steam: 0.13, buff: 0.025, csfloat: 0.02,
  skinport: 0.12, waxpeer: 0.05, dmarket: 0.05,
};

const seen = new Set();

async function tick() {
  const params = new URLSearchParams({ providers: PROVIDERS.join(',') });
  const res = await fetch(
    'https://api.skinstrack.com/v1/paid/items?' + params,
    { headers: { 'X-API-KEY': KEY } }
  );
  if (!res.ok) return console.error('fetch failed', res.status);
  const items = await res.json();

  for (const item of items) {
    if (item.liquidity < MIN_LIQUIDITY) continue;
    const buy = item.prices.reduce((a, b) => (a.price < b.price ? a : b));
    const sell = item.prices.reduce((a, b) => (a.price > b.price ? a : b));
    const fee = SELLER_FEE[sell.provider] ?? 0.10;
    const roi = (sell.price * (1 - fee) - buy.price) / buy.price;
    if (roi < MIN_ROI) continue;

    const key = item.slug + ':' + buy.provider + ':' + sell.provider;
    if (seen.has(key)) continue;
    seen.add(key);

    await fetch(WEBHOOK, {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify({
        content:
          '**' + item.market_hash_name + '**\n' +
          'Buy on ' + buy.provider + ' @ $' + buy.price.toFixed(2) +
          '  ->  Sell on ' + sell.provider + ' @ $' + sell.price.toFixed(2) +
          '  (' + (roi * 100).toFixed(1) + '% net)',
      }),
    });
  }
}

setInterval(tick, POLL_MS);
tick();