You can play with the vjust argument in position stack.
df <- data.frame(Seller=c("Ad","Rt","Ra","Mo","Ao","Do"),
Avg_Cost=c(5.30,3.72,2.91,2.64,1.17,1.10), Num=c(6:1))
df
#> Seller Avg_Cost Num
#> 1 Ad 5.30 6
#> 2 Rt 3.72 5
#> 3 Ra 2.91 4
#> 4 Mo 2.64 3
#> 5 Ao 1.17 2
#> 6 Do 1.10 1
library(ggplot2)
ggplot(df, aes(x=reorder(Seller, Num), y=Avg_Cost)) +
geom_col() +
coord_flip() +
theme_classic() +
theme(axis.title = element_blank(),
axis.text = element_blank(),
axis.ticks = element_blank()
) +
theme_void() +
geom_text(aes(label = Seller), position = position_stack(0.5), color = "white")

library(dplyr)
#>
#> Attachement du package : 'dplyr'
#> The following objects are masked from 'package:stats':
#>
#> filter, lag
#> The following objects are masked from 'package:base':
#>
#> intersect, setdiff, setequal, union
tmp <- mtcars %>% group_by(cyl) %>% summarise(tot_mpg = sum(mpg))
tmp$cyl <- factor(tmp$cyl)
ggplot(mtcars) +
geom_col(aes(x = reorder(factor(cyl), mpg), y = mpg)) +
coord_flip() +
geom_text(data = tmp, color = "white",
aes(x = cyl, y = tot_mpg, label = tot_mpg), position = position_stack(0.5))

Created on 2019-10-06 by the reprex package (v0.3.0)
Is this what you want ?