If the only thing in column E is the sales data, and if you want to average the last five contiguous entries in that column, you could use:
=AVERAGE(OFFSET($E$1,LOOKUP(2,1/ISNUMBER($E:$E),ROW($E:$E))-1,0,-5))
But if you have numeric data below the last row of relevant data, you will have to explain your set up better.