Buyer only here (at this point--I have a few odd serial number bills I want to throw on the Bay after I move, but not yet). Here is the return policy I plan to offer:
Your item will be shipped with delivery confirmation. If at any time in the seven days following delivery you decide you are not satisfied with the item, please contact me for a return. I will issue a full refund to you upon receipt of the item. Because of the unique nature of notes sold for their serial numbers, I will not be able to replace or exchange your item.
Items may not be returned due to damage that occurred after you received the item. Items returned for damage while in transit must include a photo of the damaged package before you have opened it*.
Now: what I expect from sellers I buy from. I expect exactly what I outlined above, at minimum, to wit: if I receive the item and it's damaged because of either your negligence or the negligence of the post office, I should be able to return it for a refund. If there's a problem with the item (e.g., you told me I'd be getting a coin in AU and I got a coin in F), I expect to be able to return it. I do not expect you to accept my return because I got buyer's remorse or bought two of the same date and want to return the one I like less.
*A single bill, shipped between two flat pieces of cardboard, double-taped and in one of those little bubble-mailers--if the bill is damaged in transit, that envelope better be wasted. I take good care of things I send through the mail, especially if you're paying me for them. The photo is for my protection and doesn't have to be high art--a cellphone picture is fine.