Watwat

How come that sometimes, when polling the TF2 inventory JSON, the keys in rgInventory (the IDs of items) do not match up with the IDs provided by the GetPlayerItems call?

Sometimes they do, sometimes they don't.

steamcommunity.com/id/XXXXXXX/inventory/json/440/2

The appID as well as the contextID are the correct ones in this URL.

I forgot to mention that I'm mainly interested in the market_hash_name of the items. The backpack.tf API doesn't feature all items (weapon skins and a bunch of outliers)

Outside of "building" it on my own using the quality and name attributes, one does have to use the inventory JSON, which makes sense, because as you put it, neither GetSchema nor GetPlayerItems has no concept of something like the Steam market.

Any tips on how to not get on Valve's bad side when scraping inventory JSON besides using a VPN?

 as far as I know TF2 and possibly Dota are the only Valve games where IEconService gives enough useful information to build an item's display information. For everything else, you need to use inventory JSON.

How would one go about that? Neither GetPlayerItems nor the schema say anything about an items classID. I'd rather not fetch the inventory JSON, unless I absolutely have to, for fear of getting banned.