In the UK there's a standard(-ish) form [1] the seller competes, before contracts are exchanged.

So if a seller wants to keep their smart doorbell, or their curtains, or to take all the plants from the garden - they can agree that, as part of contract and price negotiation. At the same time, the buyer gets to send people to check the property and can lower or withdraw their offer depending on what the inspections reveal.

(And yes, this is a very slow way of doing this)

[1] https://www.rlo.law/wp-content/uploads/2019/08/fc.pdf