Im sure most sellers in the UK know by now, but in case you have been hiding under the mound of books that you are selling please be aware that the postage rates and the method of calculating the postage is changing.
The new method uses size (volume) and weight to calculate the postage. Royal Mail have been sending out guides left right and centre with an easy to use guide to parcel/letter classification.
For me when I sell books I think the new postage prices will cut my postage costs massively which will be more then welcome and I should imagine this would be the same for a lot of book sellers across the UK.
Interestingly second class post will also be available up to 1Kg where it was limited to 750g before, typically most of the books I sell are in the 700g-1.2Kg range so this does give me the opportunity to make a little extra saving if I need to.
Generally I prefer first class post as I think the little extra it costs is worth the customer getting it faster and the perceived impression a first class stamp on the package makes over a second class stamp. If Ive sold a book through Amazon market place then the buyer has to pay a fixed postage price which is not reflected by the item they are buying, if the book is light to post then sellers can make money on the postage, however from a customer point of view paying about £3 and receiving a book in the post with a 60p second class stamp creates a bad first impression in my eyes anyway!.