This page is just a reference of all the known Steam APIs, I do not know how they work. Please do not email me with questions.
PUBLISHERPOSTCancelAppListingsForUser

Cancels all of a user's listings for a specific app ID.

https://partner.steam-api.com/IEconMarketService/CancelAppListingsForUser/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
uint32 Yes The app making the request
uint64 Yes The SteamID of the user whose listings should be canceled
bool Yes Whether or not to wait until all listings have been canceled before returning the response
bool Yes This was in response to a VAC ban?
PUBLISHERGETGetAssetID

Returns the asset ID of the item sold in a listing

https://partner.steam-api.com/IEconMarketService/GetAssetID/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
uint32 Yes The app that's asking. Must match the app of the listing and must belong to the publisher group that owns the API key making the request
uint64 Yes The identifier of the listing to get information for
PUBLISHERGETGetMarketEligibility

Checks whether or not an account is allowed to use the market

https://partner.steam-api.com/IEconMarketService/GetMarketEligibility/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
uint64 Yes The SteamID of the user to check
PUBLISHERGETGetPopular

Gets the most popular items

https://partner.steam-api.com/IEconMarketService/GetPopular/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
string Yes The language to use in item descriptions
uint32 NoNumber of rows per page
uint32 Yes The result number to start at
uint32 Yes If present, the app ID to limit results to
uint32 Yes If present, prices returned will be represented in this currency
PUBLISHERPOSTLearnItem

Teaches the market about a kind of item that may be listed on the market in the future.

https://partner.steam-api.com/IEconMarketService/LearnItem/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
uint32 Yes The app the item belongs to
string Yes Asset class property names
string Yes Asset class property value