Make Trade Online

I am going to Portland, Oregon for the holidays and I was excited to read on Daily Candy that Powell's bookstore (one of my must-have places to visit when I get there) is offering the opportunity for everyone to sell books online for online credit at the store, genius idea!  Here is what the Daily Candy email had:

"SELL
Your Old Books
What: Popular Portland bookstore Powell’s will buy back your old reads. Just enter ISBN numbers on the site, mail ’em in, and they’ll send you virtual store credit. 
Why: This reading rainbow yields gold. 
Where: Online at powells.com."