I have found these lens Nikon AF 70-300mm f/4-5.6G in a very good price. I am planning to use them mainly to take pictures of the moon. I am aware that this lens does not have image stabilisation. Is this a major drawback ?
If you plan on shooting a full moon on a clear night, handheld, it shouldn't be, because you can easily get shutter speeds faster than 1/500s. Remember that the moon is a directly sunlit object, and expose accordingly.
If, however, you plan on shooting a crescent moon or during a lunar eclipse then, as long as you're shooting on a tripod, it shouldn't be a major drawback to not have stabilization, but you'll want good technique, probably a cable release, and mirror lock-up, and it can certainly be helpful, so long as the stabilization is compatible with tripod use.
PS. Does anyone know is this is a decent lens?
Depends on your definition of decent, but any sub-$250 70-300 telephoto zoom is typically a consumer grade lens, and isn't exactly the kind of glass National Geographer folks use for wildlife or landscape shooting. That doesn't mean you can't get a good picture out of one, but you will need to learn how to get the best out it, much as you do an 18-55 kit lens. Stopping down into the f/8-f/16 range can help. Not shooting at the 300mm end of the lens might help. Things like that.
Also be aware that as an AF, not an AF-S lens, it will not autofocus on a D3x00 or D5x00 body.