When we added the Marketplace.tf API, we included an endpoint to set the price of one of your items. We did this so that people could more easily control their stock and adjust their prices if necessary. At the same time, we intentionally provided no endpoint that would allow a user to find the lowest price of an item they're selling -- specifically to prevent users from writing scripts to automatically undercut other users by very low amounts.
Nevertheless, users are writing scripts to automatically undercut other users. In retrospect, this is partly our fault; the inclusion of the SetItemPrice endpoint clearly sent the message that automation in this way was okay, and we haven't publicly stated elsewhere not to do so. So today we've removed access to the SetItemPrice endpoint.
Moving forward, users attempting to use scripts to automate undercutting of other users will be banned from selling on Marketplace.tf.