You dont need booking system. You need just 2 things:
1. Check list – so that owner can set what apartment have example Aircondition, parking, pool etc if someonething doesnt have to have forbidden icon.
2. Price – Price needs to be somewhere visible and bolded. Ofc it need 1 check field for price and 1 drop menu for currency (€,$).
For everything else there is contact form where visitor can contact owner or by phone.
wpgeo devs could make something like this very easy :p