Yeah this is all very very easy:

my @names = qw/andy betty craig daniel edith/;
my %reverse_lookup;
@reverse_lookup{ @names } = (0..4);

foreach $year (2004 .. 2007) {
    foreach (sort keys %people) {
        printf("%04d: %s buys for %s\n", 
            $year, 
            $_, 
            $names[ ($year+$reverse_lookup{$_}) % 4 + 1]
        );
    }
}
[download]

However, the group isn't constant. My original data introduces nancy after two years, if we suddenly do this then a latin square doesn't work.

Also, I must point out that there will be data such as sally can never buy for derek ... not after last years christmas party. So derek is on sally's list .. permanently. Even after we reset it at the end of the cycle.