A long time ago, when I was a music-obsessed teenager, I had a few vinyl records which were apparently quite rare. There was one I used to refer to, as I grew older, as my pension fund. Of course, with the coming of eBay, things changed as far as the rare vinyl market was concerned (I think I sold that particular record for about £35 a couple of years ago). As a dealer of books, including some pre-loved books, I do now and again encounter something that might be classed as rare and therefore valuable. Sometimes I get lucky and find a tasty treat in a bundle of used books going as a job lot; sometimes, particularly on eBay, someone with deeper pockets than me gets the prize: one particularly annoying incident was when I was the lead bidder with three minutes to go. I decided to go and have a cigarette, and when I got back, another bidder had offered an extra 50p and the book was theirs.
Of course, an allegedly rare book is only worth what someone’s willing to pay for it, and one of the few things I do use Amazon for is to set prices for any second-hand stock I might have acquired. Sometimes it’s worth checking more than once, as what was being offered for £10 last week might suddenly have jumped up to £50 or more as the only cheap copy has now been snapped up (probably by someone intending to relist it for £75), though more often the scarily rare book priced at hundreds of pounds has suddenly plummeted to about a fiver when a dozen more copies appeared. And, of course there’s the fact that, just because the asking price is 
astronomical doesn’t mean there’s anyone out there actually willing to pay that price.
I have Aishling Morgan’s Slave To The Machine in stock at present, and I’m keeping it at £7 despite seeing it listed on the Zon for upwards of £30. Get yourself a copy if you fancy it – it’s a filthy, fun read with a hapless heroine getting into some terrible subby trouble in a virtual reality or several.
I’ve also got a few interesting newly-unboxed vintage titles listed on eBay myself, for auction, as a way of testing the water for that type of thing now I can’t hustle and haggle with customers face to face. And the usual ever-changing range in the bookshop if you just want something to read.