Item not as described - ebay?

Hiya, Just wanted to get some opinions on this .... I bought a vintage Beatles jigsaw puzzle on ebay and it stated that it was complete. It was very expensive and when it arrived I noticed that 3 of the playing cards were missing. I enquired with ebay to ask if it would be OK for me to ask for my money back and they said I needed to contact the seller and that they couldn't help because I had paid by postal order and not paypal (I do not have a paypal account as I have had trouble with them in the past). The seller has said that she's sure the game was complete and that the cards must have got mislaid when she moved house. She said they were probably still at her ex-husband's house and that it would take some time for her to check. How long do you think it would be reasonable for me to chase her on this - I am really disappointed as I had to save for quite some time to get this piece of Beatles memorabilia. What could I say to her - I don't want to be too aggressive in asking because as I said I have no comeback with paypal/ebay. Thanks all.