Shanna Moakler Auctioning Off Engagement Ring from Travis Barker

Shanna Moakler's marriage to Travis Barker is long gone, and she's finally decided to cash in.

Shanna's putting the engagement ring she got from her rocker boyfriend back in the day up for auction, and she stands to make a pretty penny.

The 4-carat bling could be yours, if you have a minimum of $51,000 ... that's the starting bid.

She says it's a symbol of moving on ... Travis has found other ways to symbolize moving on, as in marrying someone else.

Shanna told Us Weekly, "I think it is a beautiful piece and truly was my dream ring. However, that chapter of my life is over and even though wearing it at one time brought me great joy, I hope it will find a new home and continue to bring someone as much happiness as it once did to me."

The ring's Cartier, and it's being hawked for sale on Worthy.com. She's gunning for way more than $51k ... more than twice that. She says it's worth $160k, and would love to pocket $120,000.

As for timing ... Shanna says it has nothing to do with the fact er ex just tied the knot with Kourtney Kardashian. She told Us Weekly, "I'm sure people would assume that, but I simply wanted to find a proper seller and Worthy.com is the most professional and I know will give this piece the attention it deserves."