Dilemma:    For whatever reason, you can't pay for something that you won.   

Why you should care:   Because you're a decent human being who understands that many sellers sell full time on Ebay to support a family.   We depend on you to hold up your end of the log and pay us on time, just as you depend on your boss to give you a paycheck that won't bounce.  

DO:   As soon as you become aware that you won't be able to pay on time, DO email your seller immediately with a date that you can pay.   Don't fib or fudge; just be honest.  Most sellers are willing to work with people who give a specific date that isn't more than a few days late.   Ebay allows sellers to decide how long they will wait for payment.   Some sellers require payment within 3 days, and some sellers will wait as long as a month.  Most sellers fall somewhere in the 5-10 day range.

DON'T:    Don't wait until an Unpaid Item Dispute has been opened to contact your seller.   Most sellers (including me) are loathe to work with someone who will not respond until a UID has been opened.  Ebay doesn't require that sellers accept late payments, they only require that sellers open a dispute in order to get their Final Value Fees refunded.  The sooner you email your seller, the better.

DON'T:    Don't bother making up a story as to why you can't pay.   From dead grandparents, to bailing friends out of jail, to installment payments for plastic surgery taking precedence, there is nothing you can say that your seller hasn't heard before.   It embarasses us and it embarasses you.  So just don't do it because it's not necessary and it reduces your dignity.

SAMPLE NOTE:   

Dear Seller,

Thank you for offering the widget for sale.  I am looking forward to receiving it and I am sure it will be fabulous.   Unfortunately, I won't be able to pay for this item in the time allowed.  (notice there is no excuse here, lame or otherwise.  Just a basic statement of fact).  I will be able to pay for it on (exact date).  If you plan to relist it, I completely understand.  If you would be willing to wait for payment, please know that you will have an instant Paypal payment no later than 5pm on (exact date).       Please let me know.

Sincerely,

Buyer

Do you know how many people will work with you when they get a letter like that?  About 80%.    I know I certainly would, as long as the date you can pay is within a few days after payment is due.   The other 20% are a little more hardcore about payment time frame.   Or else, your date is too far in the future and you need to bring it a little closer to the present and try again.

Dear Seller,

Thank you for your response.  I understand that you aren't able to accept a payment on X date.   The very earliest I can pay would be Y date  (or, if you truly and honestly can't pay until X, just say so).   I really would like this item and to complete our transaction.   Please let me know if you are able to bend your terms to accomodate my late payment.

Thanks again,

Buyer

And then, don't make a liar out of yourself.  Follow through.  Pay that payment on time.  Leave five star feedback.   Your seller will love you for it, and it will salvage a transaction with dignity and mutual respect.  :)

Happy Ebaying!