If you don't know what you're looking for second hand can be a nightmare especially if you have to take it to your LBS for repairs. That's why you need to know its been properly vetted like the vintage bikes that
@biggs682 offers. If you want something new then try to get to a
Decathlon store if you can otherwise
Halfords stuff is OK. Unfortunately the service experience can be a bit variable there and has been well documented on here. I think
@cyberknight is still in dispute with them over poor prep on a new bike.