You cant stop an auction when it was already started!
You are loosing reputation no one will trust you again...
of course they can it's their items and they can sell them at their price of choice or retract their current offers without having to provide you with a reason. Jurisprudence also teaches that buyer's remorse exists, so maybe seller remorse exists as well, meaning that they might also be able to ask back at a later time.
Is it illegal? No
Is it immoral? Yes, obviously they wanted more fg, but no one bit so they will try one more time.
Will I add them to my endless block list as it's bm? Also yes.