The process for getting hold of a rare book is the same process you use to sell it for more-- it comes down to buying up as much stock as you can.
Buying and selling books on Amazon can be a lot of fun. It can also take a lot of time, and it's very easy to lose money doing it. However, if you're careful, and you see the opportunities that are there, you can amass a collection of very valuable books worth reselling on Amazon and other marketplaces in short order.
Why is it that you can buy a book for less when you already own a copy? The same mechanism that drives up prices when you're trying to get a good price for a rare book is the mechanism that will drive down prices when you're buying. In simple terms, if you are the only person that owns a book, the price will tend to rise. By the same token if hundreds of people have a copy of that book, the price is going to be pushed downward. How does this help you in the real world buying more cheaply or selling more expensively? Let's get into that.
First things first-- you need to own a relatively valuable and rare book. The easiest way to accomplish that is to monitor prices and buy the book when it is within an attractive range of something you're willing to pay. You're not always going to get 90% off the value just by watching, but there are times when a very cheap copy will come through the marketplaces and you can set up a price alerts.
Once you have a valuable book, you have to be willing to lose it, as there is a chance that the only copy that you have will sell before you acquire additional copies. But in most cases you can drive down the price and purchase more copies, if you use large companies auto pricing against them.
The first step is to list your book, and I would suggest listing that book on Amazon, even though the fees are higher. Even though this book is rare, and someone searching for will likely search everywhere for it, Amazon is still the best place for it to be found. So the first step is to list your book on Amazon.
This book is not necessarily one that you want to sell, so you do not need to express what a nice copy it is. You can give it a 'good' condition and describe it in such a way that it is less likely to be bought, for example "this is a copy in good condition, the corners are dog-eared, very readable copy".
That is not so how someone would describe a collectible book, and it is unlikely to be a copy that someone else will want to buy. Which is good, because you don't want to sell your only copy. You want to gather a number of them and then restore the price to it's normal level.
The next step is to make your book the best price available by a few dollars. Once you've done that you need to monitor the competition often. The price changes will come very quickly, within a few hours. If an when you see another seller reducing their price on the same copy, you can reduce your price further. In general, I suggest doing this in steps of about $10, unless it is a very expensive book, in which case you can do $25 increments or similar.
It depends on how much time you have, but once you have a computerized algorithm chasing your price downward, you can lower and lower and lower... until you lose your nerve. Watch for the algorithm's behavior to change. They may stop at a floor, at which point you can decide if this is a price worth buying. Often they will lower the price quite far down, because the computer wants to be the best price. Take advantage of that.
There are now two listings, maybe more, that are far underpriced for the book and you do need to act relatively quickly. You need to purchase the book(s) that you've driven down, and immediately raise the price on your book so that you don't sell your book cheaply.
If your copy does sell while you're doing this, you need to be sure and buy the copy you've driven down immediately, before the price sneaks back up. You might lose a little occasionally in a trade like this, but you need to have one copy in order to continue to use this method of accumulating copies.
Ideally now you have two copies of an expensive book, and the next time you can act more aggressively in pushing prices down. Maintain just a single listing so that you're not competing with yourself and risking two copies. And now, again, try to draw the next bot into lowering their price down.
Repeat the steps above, but now you can be more aggressive, because you have two copies at this point. Eventually you are going to run out of responsive bots to manipulate. You will be able to look and see the residual price of the book, or you will be the only owner of the book, which is a great situation to be in. Once you've gathered all the copies that you can get, you can begin to sell them. But you only want to sell them for a price that is slightly less than the other (now higher) prices.
So raise your price until you are the lowest, or close to the lowest edition for the copy, and you can wait for the sale. The sale will not come quickly, in many cases it will be weeks or months away. But if you see your listing lose its position, you can bait the bot down and gather more copies.
Rare books are not being reprinted, and first printings are generally going to stay valuable. So if you believe in the book, you can continue to gather more copies. With this method you can simultaneously drive down the price of books that you purchase while getting them all into your hands.
You do have some risk, but if you have the time and you enjoy it (it can be fun) then give it a shot. There are caveats to this approach, one of which is when the lowest price is driven by a very poor condition book. Beat-up books are not valuable, almost regardless of rarity. Written-in copies or sticker-laden library books for example, take a very rare book to become valuable. And if the lowest price on an edition of a book is a lower-quality copy, you can make a decision on whether it looks like a boilerplate description of a large seller or not.
Big sellers often use a general description that covers their minimum condition, so they can save time listing. If it's actually a book that is in terrible condition, you probably do not want to buy or sell it and you probably do not want to compete on the price for its position. Your price should match or beat the lowest price of a good copy, and you cannot effectively drive down automatic prices by making your book condition sound terrible.
So if you list an acceptable copy, as opposed to a good or very good, often bots will not follow you down the price slope. So you have to find the balance between decent condition books that they want to compete with, and a collectible book that is just not that great for an actual human buyer, and dissuade them from making the purchase of your under-priced book while you gather other copies.
You get a feel for it very quickly, and again, it can be a lot of fun. If you have examples of books that you've done this with please share them at firstname.lastname@example.org.