Hi, recently I came to know about a second-hand bicycle shop in Camden town at Eversholt street near Mornington crescent tube station. They have some nice collection of second-hand bicycles; prices usually start from 50 pounds upwards. As for as i know they buy and sell second-hand bicycles but you need two piece of ID to sell them a bike. Their second-hand bicycles come from auctions and they reconditioned them before selling. Staff at Camden cycles is very helpful and knowledgeable..... but it always handy to give them a call beforehand 020-7388-7899
Enjoy your summer