A tenancy agreement does define it, but say the other party breaks the agreement. It is a lot of stress and agro, and you are alot more likely to get something back from a company trying to protect it's reputation than a guy renting his old house.
If I buy something from eBay and it goes wrong, I might get my money back if it's a good seller, but I also might now. If I buy the same item from a shop I may pay a little more but I have the piece of mind on knowing that should it go wrong it will be replaced relatively swiftly.