Here's how it works: You advertise your bike for sale on the website for, say, £500. Someone contacts you saying they want to buy it (usually for a client) and agreeing to pay the asking price.
Then they come up with some story that involves them paying you, say, £3,500 with the idea that you are to send them money in return and they'll make it worth your while. You send them the bike and £2,500 and you get to keep the £500 for your bike plus an extra £500 for being so helpful. Sorted!
Trouble is, once you try to cash their cheque, you find that it's fraudulent. So you're down one bike and £2,500.
There are several versions of this scam going around, but the one thing that they all have in common is that they seem like money for nothing and they all involve you paying out cash. Be suspicious of anyone mailing you from Nigeria (phone numbers starting +44 703) cos that's where most of these scams seem to originate.
Don't get caught out!