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.
UNDOCUMENTEDPOSTCancelBid
https://api.steampowered.com/IAuctionService/CancelBid/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
uint64 No
UNDOCUMENTEDGetBidsForItem
https://api.steampowered.com/IAuctionService/GetBidsForItem/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
uint64 No
UNDOCUMENTEDGetBidsForUser
https://api.steampowered.com/IAuctionService/GetBidsForUser/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
fixed64 No
UNDOCUMENTEDGetItemDetails
https://api.steampowered.com/IAuctionService/GetItemDetails/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
uint64 No
UNDOCUMENTEDGetUserBidForItem
https://api.steampowered.com/IAuctionService/GetUserBidForItem/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
uint64 No
UNDOCUMENTEDPOSTPlaceBid
https://api.steampowered.com/IAuctionService/PlaceBid/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
uint64 No
int64 No
int32 No