used bikes in belfast?

Hi, 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
apologise🙂), lifafa