The mortgage issuers get to decide what risks they do or do not accept and they hate unknowns. One example that often comes up is "atypical construction". Maybe you want to buy a one-of-a-kind house that some crazy architect built for himself in 1968. He didn't use any materials that are known to be unsafe, but on the other hand nobody else had ever built a home like that and no-one did since. Mortgage issuers hate that, you might find a specialist who would be willing to offer you a mortgage, but nobody on the high street will.
It's not unsellable, there are a lot of private buyers, both investors who intend to rent it out and people with significant money looking to just buy outright. For example my mother moved long after retiring, obviously she can't get a mortgage, so she paid cash. However, if a mortgage will be hard to get this makes it significantly harder to find a buyer, especially if your ideal buyer is younger and not wealthy. This can depress prices.
It's not unsellable, there are a lot of private buyers, both investors who intend to rent it out and people with significant money looking to just buy outright. For example my mother moved long after retiring, obviously she can't get a mortgage, so she paid cash. However, if a mortgage will be hard to get this makes it significantly harder to find a buyer, especially if your ideal buyer is younger and not wealthy. This can depress prices.