They have researchers, working for the production company, who find the properties; it would be a bit embarrassing if the properties were sold before the camera crew turned up; and the properties are chosen as much for the entertainment and education of the viewers as for their chances of being of interest to the people on the programme. Good and bad reactions by them are a part of the entertainment .
Otherwise, it's just like real life. Countless estate agents have the knack of showing you properties which are almost the exact opposite of what you've asked for, and at too high a price !