I research historical books on Mt. Rainier National Park and often search on-line catalogs for booksellers who consolidate a range of national and sometimes international booksellers, such as Abebooks, Alibris Books, etc. I'm currently waiting for the last two of 8 books I bought from one of them. Six arrived within a week and the other two are still "in the mail", I guess, but it's not here.
Anyway, several things I learned, which are the following.
First, don't buy a government document until you search the agency's on-line catalog and bookstore to see if they sell original print or copies of the publication. This is especially true as many are moving their old publications to free PDF publications. The USGS is doing this in that their bookstore shows if an original print is available or a free PDF is available.
Second, don't buy a copy of an article in a professional journal until you do a Google search as some Websites post on-line copies for free. Professional journals sell copies, and while they're often expensive, they're sometimes better print copies than those from booksellers.
Third, and very important, don't buy "Print on Demand" books unless you do some history of the publication and especially the seller. Some in the US, United Kingdom, India, etc. sell scanned copies of old publications, but check to verify it's an actual published document. Some sell fake books, simply reprinted Websites, often Wikipedia, as a book.
Note.--With the "Print on Demand" for some used books, be careful as I've seen many old books and documents mislabeled or misidentified, in part to hide the year of the publication. I know this because I have original print editions of these books or publications.
Fourth, be careful of booksellers who don't have a physical address, meaning they don't list one or it's a Post Office box. You can't determine if they're real or just someone working from their home. If all you have is their Website, then check it's history (check the domain name), then check on-line reviews of them.
Fifth, search the prices. Several sellers offer the book and in varying condition. This is obvious.
Ok, I'll add more when I learn them, mostly from what not to do than what to do.