Well, first, I'm not certain that comparing people's sales to the average is
all that meaningful.  Remember that in order to have an average, you must
have a high and a low value somewhere.  No matter how strong your sales force
is, unless they all sell the same, someone is always going to be top dog and
someone will always be at the bottom.  Unless you're looking for people who
are consistently at the top/bottom of their game.

Anyhow - what if we go with 2 columns for your check/X entries?  One for
week 1 and one for week 2?  And in each we compare the individual's sales to
the average of the group for that week?

Start by formatting the cells that will have the check/X entries with the
Wingdings 2 font.  That allows a capital O to display as an x and a capital P
to display as a check mark.  Then all you have to do is set conditional
formatting for those cells to turn text red if they contain "O" and green if
they contain "P".
The formula in those cells might look like this (row 2 used as example)
=IF(B2>AVERAGE(B$2:B$9),"P",IF(B2<AVERAGE(B$2:B$9),"O",""))
that compares their sales in week 1 to the group's average for week 1, while
=IF(C2>AVERAGE(C$2:C$9),"P",IF(C2<AVERAGE(C$2:C$9),"O",""))
would compare their sales in week 2 to the group's average for week 2.

Or another approach, using the layout you have, would be to compare their
individual 2-week average to the group's 2 week average:
=IF(D2>AVERAGE(D$2:D$9),"P",IF(D2<AVERAGE(D$2:D$9),"O",""))