I'll say that you may / may not want to publish what you do / deem worthy for sellers to get their FVF back.
However, I think it may help sellers understand the circumstances in which you will refund a FVF..
Situations that can happen are
a) Someone places a bid and wins or places a buy now and then makes no contact with seller to pay or cancel sale
b) Buyer uses buy now with a business seller and then cancels sale, under the distance selling regulations the business seller has to accept the buyers choice
c) Buyer uses buy now with a seller that is not a business, but the seller treats the sale as a business should do under the DSR's
d) Buyer bids or uses buy now and seller cannot fulfil the order as a business seller
e) As d, but the seller is an individual
f) As c, except the seller don't use the DSR's and marks as NPB even though they have been told by the buyer that they don't want to continue with the transaction
There may be other situations, but the above 6 in my experience cover the most frequent ones
So with all the above said, could you add something somewhere (maybe under the fees section) which states what you will consider (or give) a sellers FVF refund.