Out of Print Book Search
The best used and out of print book
search that I've found is:
Tips on Using BookFinder
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.