How illegal is this?

to send a direct lag bomb to users in quibids.com so that you can get the item to your self and profit?

Well I would consider that as a DOS so very If the government views it the same way.

Millions of users online at one time. You Dosing a few hundred of them or even a few of them wouldn't stop the rest. You would most be likely dosing the site it's self. In which case, is illegal.