Estate agent selling property without planning permission

The owner sells a property, the fact they are an estate agent is irrelevant. If the owner wants to sell, their agent does so under their instruction.

The property exists, and can be sold. Planning issues and all. The new buyer will inherit the issues also. If those issues include enforcement action, then they are buying flat that won't be there much longer.

Whether a bank will give a mortgage is up to the bank. If the loan amount was still less than the value of the house alone, ie if the flat wasn't there, they may still lend. Cash buyer can do as they like. A future buyer can do the same, and the council would presumably treat it the same and enforce removal

They can advertise it and sell it at whatever price they want. But someone would be foolish to buy unless this issue is notably factored into the price.

Excluded what? And are you talking about excluded or exempted?