used bikes in belfast?



I'm going to visit Northern Ireland in a couple of weeks and planning to
explore it on the bike.
Any ideas where can I buy a used trekking/touring bike in Belfast or in
Londonderry (to tell the truth, Coleraine would be preferable, but I'm not
expecting to get a variety of choice there), so that later I could later
sell it without losing much :)?
Unfortunatelly I cannot take my bike with me to NI.
Bying a new bike is an option I also consider, but I'm afraid I wouldn't be
able to sell it later without a noticable loss.
Thanks for any scrap of information and/or advice.
Greetings from Lodz,
Hope not to enfuriate anyone ( If I did, for whatever the reason may be, I