Calculating density within a shapefile

I'm trying to calculate density within a shapefile, but I'm fairly confident I'm doing it wrong. The idea is to figure out which geographical regions there have been the most sales by density.

Here is a link to the file that I use (testdata.shp)

library(sf)

sample <- st_read("testdata.shp")

sample$area <- st_area(sample$geometry)

density_calc <-sample %>% st_buffer(0) %>% group_by(areas) %>% summarise(`Sales (density)` = sum(sales)/sum(area))

Here are the details of the shapefile:

Geometry set for 2106 features 
geometry type:  MULTIPOLYGON
dimension:      XY
bbox:           xmin: -120.0065 ymin: 35.00184 xmax: -114.0396 ymax: 42.00221
epsg (SRID):    4326
proj4string:    +proj=longlat +datum=WGS84 +no_defs

I guess my issue is, I don't really know what is right and wrong, so I have no clue if I did it correctly.

Sorry if it's not the most extensive question, I just don't remember my high school geometry that well!