Emails from eBay members asking questions come through as randomstring@members.ebay.com which can make them hard to count as a group
The following R script re-codes them so they can be counted.

install.packages("plyr")
library(plyr)

setwd("/home/mike/Desktop")
dir()
inbox<-read.csv("inbox.TXT", sep="\t")
names(inbox)

inbox$EADD <- ifelse(grepl("members.ebay.co.uk",inbox$From...Address.), 
                      "members.ebay.co.uk" , 
                      c(as.character(inbox$From...Address.)))   

str(inbox)

f <- ddply(inbox,c("EADD"),summarize,N=length(EADD))
plot(f$N)
head(f[order(-f$N),])
str(f)

Once you've identified the biggest culprits, make a rule to move or delete them from your email.