YABU.. it's a little bit harsh if you are not out of pocket but just slightly miffed that you didn't get the item you won.
At the moment Ebay sellers are having quite a hard time of it and the constrainghts on them are high, plus Ebay seems to be geared up to make it as easy as possible for false comments. It is a sellers market and every negative no matter how unfounded counts.
I had a buyer who finally messaged me after 10 days and 1 reminder to say that they couldn't pay.
They asked if I could sell it to them for half the price they won it for.
I said I couldn't and I would offer it to the next lowest bidder on a second chance offer. ( NO MONEY HAD CHANGED HANDS AS THE PRIMARY BIDDER COULDNT PAY)
The non-paying bidder agreed to this and the transaction was cancelled. I ended up having to re-list the item (at expense to me).
The item was won at a price considerably less than before.... The winner left immediate positive feedback.
The non-paying bidder, however then, left negative feedback stating 'DONT TRUST HER'. This is now a 'black mark' against me, and does 'put off' buyers, even though the transaction was cancelled AND if I want to get it removed Ebay will not do it and I will have to go to small claims court!!! (extra expense to me), I only make enough to pay my Gas bill & mortgage, and the pool of saleable items from the loft is growing thin.
Thankfully I have a small group of regular buyers or else I would probably have to give up Ebay due to the defamation.
Saying that, the negative mark is still ther for another 6 mths and I wouldn't like to say how many new buyers have been put off.
By all means if you have been conned, swindled etc leave a negative but not if you have been refunded, (things do break , even when packaging).
If you do have a problem then contact your seller, any honest seller will reply and often compensate you for broken /INAD items.
In all go easy with that negative feedback button as most of us sellers are using Ebay to pay the bills and get penalised for any indescretion.