Items are acquired around a specific theme.
Acquisition happens after planned searches. Items are purchased and are limited in number.
Items are arranged, stored, and/or displayed in an an organized fashion. The items don't take over living spaces.
Items have a pleasurable or positive impact. Collecting them usually does not cause financial distress, nor impairment in work, social life, etc.
Source: 5th edition of the Diagnostic and Statistical Manual of Mental Disorders (APA, 2014)