There's nothing you can ask the seller to be sure it "works". The seller will tell you something and then you have to actually buy it and examine it yourself to see if the seller was being accurate. Some people also define "works" differently than others. Cameras may be said to "work" if they are functional enough to take photos, but that doesn't mean everything the camera could do when it left the factory still works.

For instance, a lens may be stuck at f/5.6. As long as you're willing to take all of your photos at f/5.6 it works "well enough". But if the camera or lens offers f/2.8 or f/16 and you want to be able to use f/2.8 or f/16, then it doesn't "work" for you. Or maybe the built in flash or flash sync connection for an external flash doesn't work? As long as you're not interested in using flash, that's not a problem.

The best you can do is ask specific questions: (These are more general than for just the specific point and shoot fixed lens camera that interests you)

• Are all of the shutter speeds the camera has functional and accurate?
• Does the camera properly stop down lenses to the set aperture?
• Does the film speed setting work properly?
• Does the light meter work accurately?
• Is the film counter accurate?
• Are any other buttons the camera has functional?

If the camera has a built-in flash:

• Does the flash fire when it should?
• Does the flash power setting adjust as it should?
• Do any additional functions, such as focus assist or pre-flash (red-eye reduction, TTL metering, etc.) work as they should?

If the offer includes a lens, then also ask:

• Does the lens focus smoothly and accurately all the way from MFD 9minimum focus distance) to infinity?
• Does the lens stop down to each aperture setting accurately?
• If it's a zoom, does the lens zoom smoothly all the way from widest to longest focal length?

You're still dependent upon the knowledge, not to mention truthfulness, of the seller when they answer your questions. A good practice on eBay and similar marketplaces is to never buy something "as is" that you're not allowed to return for a full refund if the item does not arrive in the condition advertised.