Buying a large quantity at once (often by an adult), especially for collectibles such as manga, figures, or trading cards; colloquial term for bulk/collector purchases.
子どものころ集めていたカードを大人買いして、全種類そろえた。
I bought lots of the cards I used to collect as a child and completed the whole set.
限定フィギュアを大人買いして飾る棚が足りなくなった。
I bought a bunch of limited-edition figures and ran out of shelf space to display them.
人気マンガの既刊を大人買いして、休日に一気に読んだ。
I bulk-bought the back issues of a popular manga and read them all in one go on the weekend.