In addition to being a volunteer/attendee, you will also be a book collector. You'll receive a list of 5 books and some 'spending money' (to the tune of 35 galleons) at the end of the sign up period, which you are to collect by the end of the festival (ie. by the end of Week 5.
At the start of each 'day'/week during the festival itself, a new stall will be set up, namely Flourish and Blotts, The Paper Palace, and Tomes and Scrolls. Each will 'sell' 20+ books, which will range in price from 1-5 galleons. I've picked galleons because calculations using knuts and sickles would just be a headache. The list of books sold by each stall will be posted in a separate thread. You would post below it saying which book you would like to buy, although you'll be restricted to 'buying' one book at a time, and you will only be able to buy another one from the same stall after at least 3 other people have bought a book.
There will also be a 'Trading Post' thread in which people can exchange books for other books or galleons amongst themselves. For example, Sky might post offering her copy of Advanced Potion-Making for 5 galleons. Tarma might respond trying to bargain Sky down to 3 galleons, whereas Gus might offer Gadding with Ghouls in exchange for Sky's book. Sky could then choose which offer (if any) she decides to go for. She could also hold out for another offer.
In addition to having a price, each book will have a market value, which is as follows:
Cost: 1 Galleon; Market Value: 1 Galleons
Cost: 2 Galleon; Market Value: 3 Galleons
Cost: 3 Galleon; Market Value: 5 Galleons
Cost: 4 Galleon; Market Value: 7 Galleons
Cost: 5 Galleon; Market Value: 10 Galleons
The books will be sold from the respective stalls at a discounted rate (1-5 galleons), whereas the market value will be their actual worth. The market value is what will be used for scoring at the end of the activity. This won't be for points, but we will be offering the Book Collector award for those of you who either manage to get every book on your list and/or who manage to acquire the most valuable collection of books by the end of the activity. You'll also earn beans based on the market value of your book collection. For every galleon your collection is worth, you'll earn 10 beans.