Buying a used gas grill online
I agree with JMF though the points made by infomaniac are right on...
We just bought a house as well and it is amazing how much money you find you don't have for things you need or want :-)
We were lucky, the previous owners left their old, not so good gas grill and it is usable... My hubby had to sanitize it before I would even go near it, I am onsessive about cleanliness and germs, especially being pregnant but... in answer to your question, it is only good as a band-aid til we buy the gas grill of our dreams, it probably won't last another full season... it has been left out in the rain, was not well maintained and the flames are still not even after a good cleaning but it has definitely served us well this summer... thankfully the gas line is ok...we checked it carefully but we make sure to close it immediately after use anyway.
Some advice if you want to go the used gas grill route, I would try to find one where the owner has a receipt, if possible, to gauge the age of the cue and also, it is possible to gauge the abuse/use by looking at it. They are not that easy to clean to make look like new.
Finally, use oven cleaner and scrub thoroughly, and fire it up a few times for a half hour or so prior to using it for food to burn anything else off as well as chemicals from cleaner.
I wouldn't buy a used one. It may have been left out in the rain or been handled roughly and have small cracks in the gas lines which could lead to a fire.
Buy a small weber like the smoky joe and a chimney starter, use hardwood charcoal, not briquets and you will spend around $45-50 and it will be fast, easy, and the food will taste better than on a gas grill. It will also last for years and years.
I've never bought anything on craigslist but have sold and given away many items on there.
First the grill should come with a gas tank, and hopefully there is some gas left in it for you to fire up the grill to test it.
I'd check the gas line from the tank to the grill for wear or cracks.
If you can fire the grill, turn from high to low, then low to high to make sure the flame is even and lit in all areas, because sometimes drippings can clog up some of the holes and not allow the gas to escape in that area. It can be cleaned, but its something you would want to look for.
Last, depending on the condition of what your looking at, I wouldn't spend much money for a used grill, you may want to check out the FREE section in the For Sale area too on CL.