l.df <- lapply(ls(), function(x) if (class(get(x)) == "data.frame") get(x))

library(dplyr);
file_names <- dir() # Location of individual sales files
agg_df     <- do.call(rbind,lapply(file_names,read.csv))

# Median sale per Article ID
agg_df = agg_df %>% group_by(ArticleID) %>%
                      mutate(mSlByArtId = median(AmountSold));

# Median sale per Shop ID
agg_df = agg_df %>% group_by(ShopID) %>%
                      mutate(mSlByShpId = median(AmountSold));

dl <- list()
 dl[[1]] <- data.frame(
   shopId = rep(1:4, each = 2),
   ArticleId = c(1, 1, 3, 2, 3, 2, 1, 2),
   date = 1:8,
   AmountSoled = 5
 )

 dl[[2]] <- data.frame(
   shopId = rep(1:4, each = 2),
   ArticleId = c(2, 1, 3, 2, 4, 4, 3, 1),
   date = 1:8,
   AmountSoled = 5
 )


#  dl
# [[1]]
#   shopId ArticleId date AmountSoled
# 1      1         1    1           5
# 2      1         1    2           5
# 3      2         3    3           5
# 4      2         2    4           5
# 5      3         3    5           5
# 6      3         2    6           5
# 7      4         1    7           5
# 8      4         2    8           5
#
# [[2]]
#   shopId ArticleId date AmountSoled
# 1      1         2    1           5
# 2      1         1    2           5
# 3      2         3    3           5
# 4      2         2    4           5
# 5      3         4    5           5
# 6      3         4    6           5
# 7      4         3    7           5
# 8      4         1    8           5


 df <- do.call(rbind, dl)
 df
#    shopId ArticleId date AmountSoled
# 1       1         1    1           5
# 2       1         1    2           5
# 3       2         3    3           5
# 4       2         2    4           5
# 5       3         3    5           5
# 6       3         2    6           5
# 7       4         1    7           5
# 8       4         2    8           5
# 9       1         2    1           5
# 10      1         1    2           5
# 11      2         3    3           5
# 12      2         2    4           5
# 13      3         4    5           5
# 14      3         4    6           5
# 15      4         3    7           5
# 16      4         1    8           5

 aggregate(df, by = list(df$shopId), mean)
#   Group.1 shopId ArticleId date AmountSoled
# 1       1      1      1.25  1.5           5
# 2       2      2      2.50  3.5           5
# 3       3      3      3.25  5.5           5
# 4       4      4      1.75  7.5           5