eBay and Craigslist finds

Hello.
i am new to the forum.
i ll be visiting london next week and would like to buy a fixed gear bike.if there is a flip flop availability things would be much better.
i tried searching ebay but everything seemed to expensive or too crappy.
i v seen some things on gumtree.but cant tell which ones worth.
i know a lot about road bikes and mountain bikes, but nothing at all about fixed gear bikes.
i saw some bikes like this one.
http://www.gumtree.com/london/67/37996667.html
but can't understand why this one aint good (i really dont like th colour but what else appart from that)
thanks in advance for any replies.
cheers

Try to search this forum for unipack. there is plenty of posts about how bad are those bike. And remeber they really are very poor quality bikes in every aspect,