How do Opensea competitors (like Blur) subscribe to new NFT for-sale listings?

Question

I assume Opensea's API/SDK is blocked and not accessible by Opensea competitors, like Blur.

Now, when a new listing is created on Opensea, there is no on-chain transaction, just a gasless signature by the owner of the NFT. So I assume there is no ethereum Event since there is no ethereum transaction.

So, how is Blur (and others) able to detect and display new listings almost instantly if there is no transaction?

I skimmed the Seaport 1.1 docs, but I didn't find anything obvious.

Answer 1

They are likely using the OpenSea Stream API. There used to be public documentation for it, but the link now gives a 404. However, there is still the GitHub repository for it, it seems that it is still online: https://github.com/ProjectOpenSea/stream-js

As for them not handing out API keys to their "competitors", I think that isn't true at all. They still profit from their use of their listings, so I see no reason why OpenSea wouldn't give them an API key.