Buying a used lens can be safe, depending on where you buy. B&H Photo, KEH, Adorama are all reputable dealers and have used departments. There are other dealers, but I have only dealt with these. The Nikonians "I want to Sell" forum is also very reputable for used equipment. Personally, I would not buy camera equipment on eBay, but that's just me.

While the 18-200 is a good zoom lens, there are other zoom lenses that would work well for the type of photography that you plan on doing. The Nikon 16-85mm VR lens is a very good lens. I have the Sigma 17-50mm lens, and I am very happy with it. It has a maximum aperture of 2.8, so it is faster than the 16-85mm or the 18-200 mm, but does not have the reach of either of the Nikons. For a prime lens, the 50mm f/1.8 or 35mm f/1.8 are both very good lenses.

I hope this helps.