I mean, Ozan Kabak was supposedly suggested at the end of the window this summer, but Schalke wanted closer to 25-30 for him (from what the internet rumor mongers and twitter folks suggested). Schalke is still broke, so I guess he could be an option.
I don't think selling a guy that there's playing time is really hard. I also think at this stage, you can't rely on any of the main senior central defenders they do have to be healthy. Even though Virgil was healthy for a long run, he's getting older, and has had injury issues in the past. Matip is basically made of glass, and I don't know that I trust Gomez over a full season till he can show it. I think it's more a matter of not wanting to overpay, knowing clubs who will try and do business with them will try and gouge, and knowing that the money really honestly, isn't there either.