Fly-by-night

That outfit is running a fly-by-night operation.

Source: tatoeba (68505)

Do not give your credit card number to that fly-by-night operation.

Source: wiktionary

The automobile industry rose almost overnight. Orders poured into the offices of companies already organized; new companies were formed by dozens, capitalized at millions of dollars. Fly-by-night concerns sprang up like mushrooms, flooded the country with stock-selling schemes, established factories where parts of motor cars, bought elsewhere, were assembled.

Source: wiktionary

True, it was a good advertisement at Boosters' Club lunches, and all the varieties of Annual Banquets to which Good Fellows were invited, to speak sonorously of […] a thing called Ethics, whose nature was confusing but if you had it you were a High-class Realtor and if you hadn't you were a shyster, a piker, and a fly-by-night.

Source: wiktionary