This is a (perpetual) work in progress so please mention if something feels out of date!
Below is the documentation for various blockchain payloads that Splinterlands processes and what their payload means.
This call is used to update the price of all the cards currently listed on the market, or rented out. Rented out cards are automatically cancelled and their price is updated to the price specified when they are relisted on the market when the card is cancelled during the next rental payment processing. The payload in the custom JSON field is of the following type (remember that the field must be a stringified version of this payload):