The Tuck Shop
A poster seen in the Harry Potter films advertises a Hogwarts tuck shop, apparently ran by Mrs Tubbs. The tuck shop apparently sold freshly baked Cauldron Cakes and Pumpkin Pasties, and was advertized on the Gryffindor noticeboard. Bear in mind that this is only ever mentioned in the movies, so it doesn't hold any weight in the main (book) canon.
A large sign had been affixed to the Gryffindor noticeboard, so large that it covered everything else on it – the lists of second-hand spellbooks for sale, the regular reminders of school rules from Argus Filch, the Quidditch team training timetable, the offers to barter certain Chocolate Frog Cards for others, the Weasleys' latest advertisement for testers, the dates of the Hogsmeade weekends and the lost and found notices.
Harry Potter and the Order of the Phoenix - chapter 17 - Educational Decree Number Twenty-Four - page 325 (emphasis mine)
As seen in the above quote, spellbooks are shown to be sold second-hand, and there are apparently enough for offer at a time to warrant multiple lists. Nobody reacts as though these are a new addition, so it's likely they were always there or a recurring feature. There are likely rules concerning what you can post on the board, considering Prefects are allowed to take down notices posted by students, and it would make sense if approval is required to post things on the board altogether - it seems likely that teachers would at least know about them, at the very least.
'They are the limit,' said Hermione grimly, taking down the sign, which Fred and
George had pinned up over a poster giving the date of the first Hogsmeade weekend, which was to be in October. 'We'll have to talk to them, Ron.'
Harry Potter and the Order of the Phoenix - chapter 12 - Professor Umbridge - page 206
'And speaking of your Skiving Snackboxes,' said Hermione, eyeing Fred and George beadily, 'you can't advertise for testers on the Gryffindor noticeboard.'
Harry Potter and the Order of the Phoenix - chapter 12 - Professor Umbridge - page 210
These spellbooks available for sale are never mentioned anywhere else, though, so there's unfortunately no more information than them. It also seems quite strange that nobody suggests Harry get a second-hand Potions book (rather than a new one from Flourish and Blotts) in Half-Blood Prince (unless the definition of spellbook is more literal than I thought).