Hi, sorry for the late.
Yes, I also imagine that the owner of the auctioneer contract (which implements the logic) is the seller of the object modeled by the contract.
Quoting
[But here there is a new problem in developing the DAPP, a mechanism must be created to allow sellers to show their willingness to sell something (I imagine it should be done with a new smart contract).]
Yes, you can develop new smart contracts to help you with your DAPP. You can then put them in a "Changelog" section in your final report, for example.