What's the technical name for this cognitive bias? I'm pretty sure I've seen this cognitive bias described in the literature, but I can't find its official name, so I'll provisionally call it "the used-book finder's bias", based on the following vignette.

Say you are browsing through some used-book store, looking for nothing in particular, when you spot, tucked in some corner, almost hidden, a copy of some "classic" book.  The book is in good shape, and the price is very attractive.  What a find!  You've got to have it, never mind the little voice reminding you that your reading list is already overcrowded, as are your bookshelves, and that you won't have time to read this book in the foreseeable future...

The cognitive bias I'm referring to is the intense compulsion to acquire the serendipitous find, even though one does not need it, and almost certainly will not do anything with it.

As someone who used to frequent used-book shops, I'm all too familiar with this cognitive bias.  It led me to spend a lot of money, and acquire more books than I could even store, let alone read.  (Eventually, I wised up.)

Mind you, the vast majority of the books I found this way were not really hard-to-find; I could easily get through other means (e.g. the library) if I ever really needed to read them.  Moreover, if I had just paid full price for brand new copies of the very few among those books that I ultimately read, I would have still saved myself a lot of money.

The principal element of the bias, seems to me, is the strong feeling of good fortune one feels in such a situation.  Conversely, one feels strongly that one would be a complete fool not to take advantage of such a lucky event.
I am not entirely sure about the proper scientific names but I think your issue revolves around buying things because they are


cheap;
hard to find.


The first is a notorious reason to buy stuff; the impulsive purchase of goods, simply because they are advertised as 'buy two, pay for one!'. This bargain hunting can indeed become pathological and addictive (source: Psychology Today).   

The buying simply because a product is hard to find is a bit harder to frame in behaviors, but it is indeed a common phenomenon that people tend to buy stuff because they are exclusive (#55 here). Also, perhaps you simply wish to buy it just because you can (#26 here). 

Further, buying stuff in general makes you feel good as it activates the reward circutry in our brain. As a consequence buying stuff can become an addiction (shopaholics).

There is actually a disorder named compulsive buying disorder. It was included in DSM-III, but not in later DSMs. It's not strictly defined but it has been associated with addictive disorders, obsessive-compulsive disorder, or mood disorders (Black, 2007).   

Reference
- Black, World Psychiatry (2007); 6(1): 14–18