i think the luxury portal does layaway, but with a sizeable non-refundable deposit any reputable re-seller would likely consider your request for layaway. http://www.the-luxury-portal.com/
As a matter of fact, i sold my first birkin on layaway this past summer when the economy started getting bad. I took a 30% non-refunable deposit and then gave her 45 days to pay the balance. we documented it all with email and used wire-transfer. it worked out beautifully, but she had 100% positive feedback as a buyer and so did i as a seller! btu it does require an element of trust!