I've bought two lenses used, a 35mm f/2 from a local reputable camera shop, brought my Nikon D80 with me to try it out and it shot great pictures, I actuated the diaphragm blades with the aperture lever to check for slop, and gently shook it side to side to hear if the AF mechanism was severely loose (and depending on lens model you might hear a little tapping which is normal by design). So far I've shot some great pictures with it.
The second is a 50mm f/1.8D I bought off eBay for about $50 less than retail, I was in contact with the seller who responded to my questions promptly, had 100% feedback and I had no reason to distrust him. When I got the lens if functioned perfectly. You just have to be weary and selective. And if it sounds too good to be true, well, you know it probably is. And if you have the chance to try-before-you-buy with your own camera, do it!
Thing is with lenses, you can probably find the same deal elsewhere so don't be in a hurry to jump all over a good deal.
As for SLR preference, there really is no "better" brand so to speak. As a previous poster mentioned, glass lasts longer than any body. I personally went with Nikon after using my sisters D80, I really liked the menu and the ease of use, plus the top window readout among other features led me to buy the D80 over the D40X. As for Canon, I tried using a friend's XSi I think it was, and it was too cumbersome and hard to figure out on the fly - I owned a Canon camcorder in the past and I didn't care too much for the menu layout on that either. The Nikon fit me well. I also have the 18-200mm VR lens as well and it really is a great all-around lens, but I can tell a difference in the crispness of pictures shot with my prime lens over the telephoto.
