I've noticed several listings from this agency popping out on FlatFox with very low prices, too low. Could it be possible that they can constantly offer flats 30% (or more) cheaper than similar listings from other agencies, even in the same building? What's the catch... Anyone can confirm actually renting a property managed by this agency?
Probably very short leases which cannot be renewed as the building is set to be demolished or re-vamped at a later point in time (look at exit date on the lease )
To answer your specific question, Schaeppi Grundstucke is an established real estate management company (also offering other real estate services) and has been active for years in Switzerland.
I don't have direct experence with them (good or bad), but they are a well known established company.