It depends. Buyers need to live up to their contractual agreements. That includes those contained in the UA to which they have agreed when signing up to use the site. They have agreed to pay for what is purchased on their account.
Personally, if the item hasn't been paid for and thus hasn't been shipped, I would tend to cancel the transaction for them and immediately block them from bidding or buying any items on any of my accounts. However, in the case of an auction I might consider how the cancellation would appear to other bidders. Especially if there had been any serious competitive bidding while the listing was active.
Of course, the use of filing a NAD upon receiving an item which one had requested a cancellation on would, I believe, be abuse of the MBG unless the item was truly not as described. Unfortunately, eBay doesn't offer much in the way of defense for sellers against false NAD disputes. Pity, there was a time when there was, at least, the pretext of an investigation. But I suppose integrity just isn't what it use to be. Of course, refusing to agree to cancel a transaction on request may be just a useless stand, but "if one doesn't stand for something they will fall for anything" or so I believe the old song goes.
"It is an intelligent man that is aware of his own ignorance."
