Horizontal Barplot in ggplot2

ggplot(df, aes(x=reorder(Seller, Num), y=Avg_Cost)) +
  geom_col()

This might be an alternative

ggplot(df, aes(x=reorder(Seller, Num), y=Avg_Cost)) +
  geom_bar(stat='identity') +
  coord_flip()

Without stat='identity' ggplot wants to aggregate your data into counts.

As off ggplot2 version 3.3.0 (March 2020) the direction is deducted from the aesthetic mapping. Hence we can simplify @Justin's and @ungatoverde code to

library(ggplot2)
ggplot(df,
       aes(x = Avg_Cost,
           y = reorder(Seller, Num)
           )
       ) +
  geom_col()

Reference: https://www.tidyverse.org/blog/2020/03/ggplot2-3-3-0/#bi-directional-geoms-and-stats