Why is a seller not obligated to fulfil a sale contract? My bid has been cancelled & item removed

[saynt1]

I had bid on an item, and with no warning and under 22 hours to go, I received an automated email telling me my bid had been cancelled and to check the item listing to see why. Apparently the item 'was no longer available' despite no warning in the original listing that it had been listed as available for sale elsewhere. The seller had made no attempt to contact me directly to offer any explanation.

How is it that a buyer must honour the sale contract they enter into by bidding, but a seller doesn't necessarily and can end a listing at their discretion (and what's to stop them doing this because they're not getting the response they wanted?)

Very disappointed.