Ask the owner questions. Then I advise you not to buy the flute. The owner should let you play the flute just like any other salesman. While playing the flute, no matter how you are purchasing it, you should always follow the three K'S. Check for squeak's, leak's, and clunk's. Squeak's happen while changing notes.
Leaks are leaks in the pads or the keys.
When pressing one key down, sometimes others go down too. Make sure those other keys go down all the way, so you don't run into problems along the way. Clunk's are the sound the flute makes when fingering. All instruments make this noise, but make sure that the noise isn't too loud. Ask the salesman or catalogue company if the flute is tuned to 440 or 442.