you'd be surprised how many collectors would buy a book from, say, ken sanders for $1500 even when a search on abe turns up the same book for $1000. the "prestigious" booksellers get a premium from collectors who want advice on what to buy next and just want to own it, rather than search for it (so they pay a bookseller to search for it for them and offer whatever price). i think, of course, that this is insane, but it happens pretty often.