Recently, at work, I needed to find a full set of a 10-volume work. All I had found using Google Books was individual volumes from the set, with links to various online booksellers. The best price for each volume, listed on Amazon Marketplace, was $23.00 plus S&H. Oddly, AbeBooks didn't even make Google's "Buy this book" list, as it usually does; I assumed this meant AbeBooks didn't list it.
After listening to my whining excuses for about 2.5 seconds, my boss told me to call a certain expert. I won't tell you the expert's name, though, because I don't want him to become too busy to take my calls in the future. The expert gave me a tip which I will henceforth keep in my bag of tricks - a tip I now pass on to you.
First, go to AbeBooks. Why? Because, unlike some book sites, they have an Advanced Search page with some spectacular gizmos on it. You might even want to bookmark that page so you can go straight to it.
Second, where it says *Title: type the exact title in quotes, such as "The Story of Modern Science" (using a random 10-volume set as an example). This will filter out squillions of "approximate" search results that will only slow you down.
Third, and this is where my eyes were opened, grab the "Sort Results By:" pull-down menu. The default selection (very reasonably) is "Lowest Price." Change it to "Highest Price." This will move complete sets of The Story of Modern Science to the top of the list of results, from which you can very quickly scan down until you find a reasonably-priced bookseller, with a reasonably high bookseller rating, shipping from a location that isn't too far away for reasonable shipping charges and delivery time.
I ended up purchasing the last set on the list before it started going into offers of "Volumes 1-6." It cost $90.00 - plus an indefinite amount of S&H, since you can bet it will be more than the bookseller's default $4.00. I reckon that's a much better deal than $230.00 plus $40.00 S&H for buying each volume separately. But I'm not the expert. I'm just passing a tip along!