Value of your listings

Puts backpack items on the market. Every listing holds exactly one copy: each group in `listings` is a `price` in cents and the `items` to sell at it, and every copy in the group becomes its own listing. `items` holds backpack item ids (the `id` from `GET /public/v1/backpack`) with the quantity of each to sell, so `amount: 5` creates 5 listings. The response lists every listing created. A commodity backpack row is a quantity rather than a single copy, so the same `id` may appear in several groups to sell copies at different prices; the groups draw from one pool, and asking for more than the row's `amount` in total fails with `INSUFFICIENT_ITEMS`. At most 50 groups per request, 50 items per group. Send `price_decay` instead of `price` on a group to list it with an auto-decaying price: it starts at `start_price` and falls to `end_price` over `total_hours` (24-168). To sell something still sitting in Steam, use `POST /public/v1/sell` instead. Accounts have a ceiling on how much they can have listed at once; past it, listing fails with `LISTING_LIMIT_REACHED`. Contact support if you are hitting it. While listing is switched off site-wide, the call fails with `LISTING_DISABLED`.