The best used and out of print book search that I've found is: 
 

Tips on Using BookFinder 
by Wonsug Jung 

It's not a book store, but just a search engine. It's searches however are not complete, so you might get a fuller result when you search with an accurate book title or author name than when you try with a broad keyword such as just "Sargent". 

I first thought a broader keyword would result in, not so much accurate, but more search results, but very strangely, it's not the case with this bookfinder's. So when you have a book you want to buy, don't make a decision to buy it at your first search result, but try multiple searches.  You may find a cheaper and better copy. 

• Search first with booktitle
• Then search with author