I wanted to fetch all the NFTs that are listed for Sell for a particular Auction House that was created using the Metaplex Auction House Contract
When creating the sell listing, you must create a ListingReceipt in the same transaction as the sell listing itself.
The ListingReceipt receipt (as well as other receipts) is searchable as the struct has a field for "auctionHouse"
If you are using the auction house client (https://github.com/metaplex-foundation/metaplex/blob/master/js/packages/cli/src/auction-house-cli.ts) as a reference implementation, please note the following:
auction-house-cli "sell" command does not generate a ListingReceipt. If you want to create those, you need to add the receipt create instruction to the sell instruction that is created by auction-house-cli. Note that the receipt create instruction must appear straight after the sell instruction and they both must belong to the same transaction.
Please see this link for more reading under "Auction House Receipts":