I'm sure I've cancelled a listing, even though it had bids on it as well.
You've got to remember, that people will usually try and sell their items through other channels as well, such as Loot; Preloved; local newspaper; local supermarket advert board .... etc.
If you end up selling your item elsewhere; Ebay can't force you to sell something you no longer have.
When I advertise something on Ebay, I usually add a disclaimer at the end, " This item is being advertised elsewhere and as such I reserve the right to cancel this listing at anytime".