This doesn't look the most kosher of adverts but not everyone on Gumtree is a crook. I've bought lenses and flash gear from it, and with any luck will be selling a 300/2.8 on it tonight. There is no harm in trying to contact the seller I guess. Try and suss him/her out a bit before exchanging.
There was some advice above about only meeting at his house. While I could see the logic in that, and have done it myself, thinking about it I would rather meet somewhere public and neutral (and with a big mate in tow).
This is my favourit current add there:
http://www.gumtree.com/london/28/45834328.html
look at the shot of the body and lenses. Some great Photoshop work there.