Where can I buy lots of books online without having to pay all the individual p&ps?

[Hexagonball]

I want to buy different books (about 100) and I don't care what condition they are in as long as they are readable. where should I go online?

Amazon Prime - can get a free month's trial? Items have to be Prime eligible though.

eBay - sometimes it's free delivery or if you do 'buy it now', you can often negotiate price with the seller?

The Book Depository is pretty cheap and has free delivery on everything - I think they'd be new books though.

Is there anything suitable at The Book People? They have fiction collections where you get a few books for a set price, which works out cheap. Or can you buy second hand ones by the metre if it's for decoration and you don't want particular titles?

Book Depository and Wordery both have free delivery.

For second-hand books the Oxfam online shop is good - there is only one p&p charge per order, even if the items are being sent from many different Oxfam shops.