tmp$ mkdir -p {a,b}

tmp$ touch {a,b}/{a,b,c,d,e,f,g}

tmp$ touch b/notme{1,2,3}

tmp$ ls a/*
a/a  a/b  a/c  a/d  a/e  a/f  a/g

tmp$ ls b/*
b/a  b/b  b/c  b/d  b/e  b/f  b/g  b/notme1  b/notme2  b/notme3

tmp$ (cd a; for i in *; do [ -e ../a/$i ] && echo delete ../a/$i; done)
delete ../a/a
delete ../a/b
delete ../a/c
delete ../a/d
delete ../a/e
delete ../a/f
delete ../a/g

tmp$ (cd a; for i in *; do [ -e ../a/$i ] && rm -v ../a/$i; done)
removed `../a/a'
removed `../a/b'
removed `../a/c'
removed `../a/d'
removed `../a/e'
removed `../a/f'
removed `../a/g'

tmp$ ls a/*
ls: a/*: No such file or directory

tmp$ ls b/*
b/a  b/b  b/c  b/d  b/e  b/f  b/g  b/notme1  b/notme2  b/notme3

# as you should see, we've deleted from b what already exists in b