Petra Ecclestone -- I'm Done With L.A. ... Wanna Buy My Home for $100 Mil?

Petra Ecclestone -- the fashion designer and daughter of a Formula One billionaire who plunked down $85 Mil on Aaron Spelling's mega-mansion just 3 years ago -- wants to unload the property, but ONLY to someone who will pay more than the most expensive house in L.A.

Real estate sources tell TMZ ... Ecclestone and her husband are moving back to London and want to unload the 57,000 square foot Holmby Hills estate. She's given 5 top L.A. realtors a secret listing ... aka a pocket listing.

Petra -- who came to L.A. hoping to be the next big socialite thing -- bought the mansion from Candy Spelling for $85 mil. Petra spent a fortune redoing the joint, and now she's got a price ... and it's astronomical.

Petra wants AT LEAST as much as the most expensive property in L.A. -- it's called Fleur de Lys, and it sold in March for $102 MIL. To give you an idea ... it has a ballroom that accommodates 650 rich souls.

We're guessing part of the problem might be the fact that Petra and her hubby have one kid, and maybe 57,000 square feet is a little too big.