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.
PUBLISHERGETCanTrade
https://partner.steam-api.com/ISteamEconomy/CanTrade/v1/
NameValueTypeRequiredDescription
uint32 Yes That the key is associated with. Must be a steam economy app.
uint64 Yes SteamID of user attempting to initiate a trade
uint64 Yes SteamID of user that is the target of the trade invitation
PUBLISHERPOSTFinalizeAssetTransaction
https://partner.steam-api.com/ISteamEconomy/FinalizeAssetTransaction/v1/
NameValueTypeRequiredDescription
uint32 Yes The app ID the user is buying assets for
uint64 Yes SteamID of the user making a purchase
string Yes The transaction ID
string Yes The local language for the user
GETGetAssetClassInfo
https://api.steampowered.com/ISteamEconomy/GetAssetClassInfo/v1/
NameValueTypeRequiredDescription
uint32 Yes Must be a steam economy app.
string NoThe user's local language
uint32 Yes Number of classes requested. Must be at least one.
uint64 Yes Class ID of the nth class.
uint64 NoInstance ID of the nth class.
GETGetAssetPrices
https://api.steampowered.com/ISteamEconomy/GetAssetPrices/v1/
NameValueTypeRequiredDescription
uint32 Yes Must be a steam economy app.
string NoThe currency to filter for
string NoThe user's local language
UNDOCUMENTEDGETGetExportedAssetsForUser
https://api.steampowered.com/ISteamEconomy/GetExportedAssetsForUser/v1/
NameValueTypeRequiredDescription
uint64 Yes SteamID of user
uint32 Yes The app to get exported items from.
uint64 Yes The context in the app to get exported items from.
PUBLISHERGETGetMarketPrices
https://partner.steam-api.com/ISteamEconomy/GetMarketPrices/v1/
NameValueTypeRequiredDescription
uint32 Yes Must be a steam economy app.
PUBLISHERPOSTStartAssetTransaction
https://partner.steam-api.com/ISteamEconomy/StartAssetTransaction/v1/
NameValueTypeRequiredDescription
uint32 Yes The app ID the user is buying assets for
uint64 Yes SteamID of user making a purchase
string Yes The ID of the first asset the user is buying - there must be at least one
uint32 Yes The quantity of assetid0's the the user is buying
string Yes The local currency for the user
string Yes The local language for the user
string Yes The user's IP address
string NoThe referring URL
bool NoIf true (default is false), the authorization will appear in the user's steam client overlay, rather than as a web page - useful for stores that are embedded in products.
PUBLISHERGETStartTrade
https://partner.steam-api.com/ISteamEconomy/StartTrade/v1/
NameValueTypeRequiredDescription
uint32 Yes That the key is associated with. Must be a steam economy app.
uint64 Yes SteamID of first user in the trade
uint64 Yes SteamID of second user in the trade