Referencing this answer.
Are buy and purchase synonyms in every aspect/context of paying money?
What I thought that these terms were unit-based:
- if you pay for a single unit (1 cigarette or 1 pkt cigarette), then it's buy
- if you pay for bulk (a truck load of cigarettes), then it's purchase
In other words, purchase is for wholesale, buy is for retail.
Please correct me if this is completely wrong. Please provide the word for the situation for the same context I described.