unfortunately suz, you're in a bit of a tight bind with this one.
i've had quite a bit of experience selling event tickets and always use the "immediate payment" option for all of my listings. from experience, if you do happen to accept a best offer, then you're allowing the person to bypass that requirement, as it becomes "sold" as soon as you accept, but as you're now aware, the buyer isn't necessarily able to pay for the item at that same moment. the best option in these circumstances when it is close to the event date, is to change the price of the item listing to the price you've been offered (considering you're willing to accept it anyway) and then the person will have to buy and pay for it immediately.
the options aren't that great now for you. you can either re-sell the tickets, and then cop negative feedback from the buyer for not being able to supply the item to them should they pay before the event.
you could also cancel the purchase as not having the item in stock, but then there's a bit of an ethical issue in that you do still have the item and are going to resell it. this would give you a defect.
if you make contact with the buyer before the event and they say they don't want to buy anymore or aren't intending on paying, you could try and talk them into allowing you to cancel the listing with the reason being that the buyer no longer wants the item - though, this must be confirmed by the buyer.
other than those, the only other options really are to sit and wait until the event time and see if the buyer pays, and possibly lose the use and monetary value of the tickets, or do that, but then use the tickets for yourself (if you're in the same city / state).
if anyone else has any suggestions that i've missed for possible options the OP can take, then please chip in ๐
