Most of my research started before there were on-line used bookstores or used book service (Abebooks, Alibris, etc.) - now my main source - or on-line library catalogs, either through world catalog services or the university's own on-line catalog.
What I've learned is a lot about researching and identifying good buys, or at least sources which offer original edition versions for a reasonable or market value. What I've also learned is where not to buy from on-line sources, which is my point here, again.
DO NOT BUY "Print on Demand" books.
I say this because almost all of them are public domain publications, easily available for free with a little research, namely looking up the publisher, usually a government agency, a research or affiliate organization, or similar sources.
This is where Google, or your favorite seach engine, comes in handy where you can search for the source, and through their Website look for "publications", and likely find the same reports, articles, etc. to download for free.
In many of the cases of print on demand, the people at the source have done their research, downloaded and compiled the material into a single document, if it isn't already a single document, and the put it on the Web, usually for inflated prices.
In some case, such as historical documents which are available on-line, they're scanned them and the produced them into a single document. I've done this for many government documents for my own research to put on my iPad instead of carrying around lots of historical documents.
The only historical documents I've found that can't be found on-line are few, such a organization newsletters or publications not worth scanning for on-line access, but are available from their library, sadly always in person.
I've found this with the Mountaineers and their publications. I have a list of articles I want to make copies but are only available from copying the original volumes in the library in Seattle, which has limited days and hours for public (always with a person present).
It's on my to go list for Seattle some day to stop by and make copies along with their bookstore (am a member too). Outside of that there isn't anything I can't find through various resouces to purchase a real, original edition or find a free resource to download.
And yes, if you have some publications I don't have or have some you want to find, I'm always interested in finding new material or helping people find material.