It gets worse: When I finally did find a seller who allowed me to order the part, they accepted my order -- and then came back later saying that, oh no, actually they didn't have it either, but would I like to convert my order into an order for something else? (Fuck off.)
I still don't have the part.
The thing is that while these automatic pricing scripts exist, they aren't in enough widespread use to do what you're suggesting.
So no unlimited risks there.
Not to mention that in order to make money over normal practices, you have to a) find books that are being sold at less than wholesale price (~60%ish RRP), and b) find a book that no-one wants to buy at your competitor's super-cheap rate that they will buy at your more expensive one - you're going to have to be in for the long haul to do this.
It's not unethical because the buyer in this case can't really buy up all of the supply and therefore is just looking for arbitrage opportunities or opportunities to exploit weaknesses in a competitors business model.
The opportunities, over time, are very limited though:
1) You can't actually buy up all of the supply. If you were able to buy all of the listed supply and then reset the price you'd soon attract other sellers who had previously unlisted supply.
2) If there was real demand for an extremely high priced book I'd guess the author or publisher would be releasing a new version to meet the demand.
3) Although some printing of books are considered to be collectible, that market is limited. Generally the value of the book is the information it provides, which means that as the price rises the (perhaps less informative) substitutes become more appealing to buyers.
4) If you exploit a competitor's pricing algorithm enough to make them notice, they'll just change their algorithm. In this case, perhaps they don't sell you the books or they don't respond to your prices.
You might be able to make a few bucks on this strategy, maybe a few people could even eke out a living, but it would be risky... they'd earn it.
The calculations would have to consider the probability of not getting the book under the price of sale, the probability of not getting it over price of sale (investing the difference in avoiding a bad rating), and the chance that you won't receive a bad rating just for reimbursing it under whatever excuse. You can also ponder taking a bad rate eventually.
In other markets you'd have to basically get the book at any cost, which would make it really risky.
Maybe if you have a slightly bad reputation on Amazon that will keep people from buying your book. Maybe you can purchase the book locally and send it to the buyer in a pinch. Maybe you can purchase the book from the actual seller and just use the buyer's shipping address. Etc.