Do NOT apply an ISBN to titles which were published before the advent of ISBNS, as the information that is recorded in the eBay catalogs is generally WRONG!!! Wrong publishers -- wrong page counts -- wrong publication dates -- sometimes even the wrong authors! And, yes, even the wrong book titles!!!
ISBN numbers were originally designed to apply to PARTICULAR editions; and, obviously, may not apply to books published before the advent of ISBN numbers.
(And publishers retire ISBNS regularly, and re-use them over and over again for other, non-related titles -- I used to work for a publishing firm, and we did this quire often.) And yet I see several eBay sellers casually branding 1940s paperbacks with ISBN numbers, not bothering to notice that the information from that ISBN bears little or no resemblance to the actual book which they are attempting to sell.
The most egregious use of an ISBN that I have noticed was a seller who applied a modern ISBN to a 1940s paperback edition of a Raymond Chandler novel, not noticing that the ISBN applied ONLY to an edition that had included an introduction by a particular individual -- who had not yet been BORN at the time of the 1940s edition that was pictured!
Keep in mind -- the seller is responsible for ALL information provided in the listings; and, by using the incorrect information provided by the eBay catalog, we sellers are promoting a false listing.
Sell on the safe side, and only use the information provided by the book that you are holding in your own two hands -- at least then you know that the information is correct!
