The best place for you to sell used books would be to make use of websites that are available for this purpose, as there are not many local shops that would buy used books from you.
In order to get the most profit out of your books I would advise using the well known public sellers sites such as eBay and Amazon. With both of these website you can set the price that you wish to sell the book for and also charge a postage fee to save you from paying it.
With eBay there is also the option for people to bid for the book so there is the chance that you could earn even more money.
The problem with sites such as these however is that they might be quite time consuming to set up and there is a small fee to use the sites to sell things. Especially as there is no guarantee that your book will definitely sell.
There are many other websites out there that will directly buy a book off you, and often will pay for the postage as well by giving you a freepost stamp. However with many of these they will give you hardly any money for the book, in some cases less than a pound, which may not be worth doing unless you have a lot of books to sell.
Some of these website are listed here:
http://www.webuybooks.co.uk/sell-second-hand-books/
This would be the website I recommend you using as it is the most simple to use. You simply make an account, enter all of your books and it will tell you how much they are buying it for, then you can send as many off as you want all at once and receive the payment for it.
This is the best website to use if they type of book you wish to sell are textbooks.
Another website that you could look into.
