fly-by-night(1)

[fly-by-night(1)] {adj.} Set up to make a lot of money in a hurry,then disappear so people can't find you to complain about poor work,etc.; not trustworthy; not reliable.

Mrs. Blank bought her vacuumcleaner from a new company; when she tried to have it fixed, she foundit was a fly-by-night business.