What's the technical name for this cognitive bias?
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