most rented property in germany is owned by large firms who make renting out homes their business. for them, that is not an issue. most homes that people buy for themselves are not even available on the rental market. as you say, people who want to buy a home for themselves will not consider rented property. i believe that is true in germany too. if it happens then it is a rare edgecase, and they will be able to find out in advance if the tenant is willing/able to move out.