This sucks!!!!  

While therapeudic, venting here doesn't get you the clock
Whereas basic electrical skills might have.
I imagine the seller wasn't counting on the 'difficult' removal--- had you demonstrated a self-reliance with things electrical, they may have allowed you to remove it.
Put yourself in the seller's place, during a busy estate sale...

I tried
I tried

bless you!!! been there ...
bless you!!! been there happened to me with a perfect Nelson bubble lamp, perfect with no yellow color or holes, the lady said the new owner wants to buy it so we will not be able to sell it to you for the for the 50 bucks that we had on it , ...........so bad so sad.
makes you pissed off for the rest of the day!!!!!!

There's always breaking and e...
There's always breaking and entering. Assuming there's no video surveillance, it would be easy to get away with. Head back there tonight with some tools. BE CAREFUL and make sure you disconnect the power at the fuse box. Then report back to us tomorrow and tell us how you went. Good luck.

My Story
I have a long standing desire for the Thruston "saucer" (multitude of names of this lamp) tripod lamp. Went to an estate specifically for this 3ft floor model. Spent 3 hrs in the shivering cold, walked in ready to buy the lamp. When the lady said..."Sorry, family decided not to sell". Everything else was pure overpriced junk. Peeved no doubt.
I have NO faith in estate sales....