We have them here in Syd as well. There held at the Sydney UNI or at the Randwick racecourse. Looking at your pic there is no way our one is that big I could be wrong, same layout with books on tables. I like to go the first day and check everything out, There never seems to be much TP or DW stuff, there is always plenty of people. I have got some sweet bargains over the years, I go for the horticultural book I've amassed a healthy library of Tree books, they have a bit of age to them and some of the names have changed, but they take pride of place on the shelves under my Pratchett collection. Just a hint if you go during the last hours of the sales last day all the books are half price.
There is this huge bookstore in Newtown (SYD), called 'Goulds' it has literally tons of bookshelves cluttered with books that are placed in their specific section, but in no particular order and boxes on the floor. I've spent hours there, and the place stays open till midnight. Their fantasy and sci-fi section is well ordered though.
As for bribing book shop staff I wouldn't want to deprive other Prattchett fans the thrill of coming across that perfect gem for their collection, All the fun is in the hunt and capture. Any way I'm sure most of the 2nd hand stores already have their customers wishlist put aside.