Understand and safely use the Steam Inventory, Trading, and Market APIs

Steps

1. Fetch a user's inventory for a specific game with GET https://steamcommunity.com/inventory/<STEAMID64>/<APPID>/<CONTEXTID>?l=english&count=5000 — CONTEXTID is typically 2 for tradable game items; this endpoint is public if the inventory is not private.
2. Parse the response: assets contains item instances (assetid, classid, instanceid, amount); descriptions contains item metadata keyed by classid+instanceid — join them to get named, described items.
3. To initiate trades between users, use the Steam Trade Offer API (IEconService): CreateTradeOffer requires a Partner SteamID, a JSON trade offer containing items from both sides, and a trade offer URL token from the recipient.
4. Monitor trade offer states with IEconService/GetTradeOffers — offers cycle through states (Active, Accepted, Countered, Expired, Canceled, Declined, InvalidItems); only Accepted means the trade completed.
5. For Steam Community Market interactions, note that Valve does not provide an official public market API — the market listing endpoints observed from the web client are unofficial, undocumented, and subject to change or blocking.
6. When using trade offers in an automated or bot context, the sending account must have Steam Guard Mobile Authenticator active and may need to wait a trade hold period (up to 15 days for accounts without the authenticator or with recently changed credentials).