Analysis
NAICS 45951 is the 2022 NAICS industry for used merchandise retailers, part of 4595 (Used Merchandise Retailers).
It has only one more detailed code, 459510, which covers exactly the same activities; the definition, index entries and statistics are on that page.
County Business Patterns publishes no national count for it, either because the code is out of the survey's scope or because the figures are suppressed to protect individual businesses.
Official definition
See industry description for 459510.
Source: US Census Bureau, NAICS 2022 Manual.