RGS Book NOT available

Regarding the book NOT being available. I have had the same experience with abebooks and Amazon used books. I suppose they are put on line, sold within the store and no one goes back to delete the on-line entry. I have had the seller tell me that they are unavailable 50% of the time. It makes you suspecious, especially when they contact you a month later, claiming they found a copy but the price is now much higher. I bet 99% of the booksellers are honest and legitimate, so I will surmise that it is just a mistake that something is left on line but is NOT available.