Step one: Open a terminal (Cmd+Space, type Terminal, enter)
Step two: Paste this:
perl -wlnE 'next unless /buy ([0-9]+) (.+) for \d+ each from/; $x{$2}+=$1; END { say "Purchases: \n", join "\n", map sprintf("%40s (%d)", $_, $x{$_}), sort { $x{$b} <=> $x{$a} } keys %x }' /Users/*/Library/Application\ Support/KoLmafia/sessions/*20171228*

Step three: get output like this:
Purchases:
                     Knob Goblin seltzer (1493)
                  ointment of the occult (100)
          tomato juice of powerful power (100)
                       philter of phorce (38)
                        ectoplasmic orbs (30)
                       thin black candle (30)
                        lotion of stench (20)
                    lotion of sleaziness (20)
                       lotion of hotness (20)
                         Ben-Gal™ Balm (15)
                       Bowl of Scorpions (11)
                    lotion of spookiness (11)
                      can of black paint (6)
                 chewing gum on a string (5)
                love song of icy revenge (5)
                        glittery mascara (5)
                          big slimy cyst (4)
                        Louder Than Bomb (4)
           Knob Goblin pet-buffing spray (4)
                      lotion of coldness (4)
                        oil of expertise (4)
                         cuppa Frost tea (3)
                       milk of magnesium (2)
                 black sheepskin diploma (2)
  Xiblaxian encrypted political prisoner (2)
                 Knob Goblin nasal spray (2)
                         eldritch elixir (2)
                         black facepaint (2)
                     Black Body™ spray (1)
                 Tales from the Fireside (1)
                           sponge helmet (1)
           love song of naughty innuendo (1)
                       natto marble soda (1)
               Gene Tonic: Constellation (1)
                              grapefruit (1)
                 ancient medicinal herbs (1)
                              pixel star (1)
                        Gene Tonic: Fish (1)
       love song of disturbing obsession (1)
                        cuppa Insani tea (1)
                       collapsible baton (1)
                            moxie magnet (1)
                         pirate brochure (1)
                      Gene Tonic: Horror (1)
                                 oil pan (1)
                             tennis ball (1)
         blind-packed die-cast metal toy (1)
                  Ancient Protector Soda (1)
                     jumping horseradish (1)
            Mesmereyes™ contact lenses (1)
                         sea salt scrubs (1)

Step 4: if you look at the end of the command pasted on step 2, there's today's date -- change it to get data for other dates, or for a bunch of dates at the same time.  For example, this does all of December:
perl -wlnE 'next unless /buy ([0-9]+) (.+) for \d+ each from/; $x{$2}+=$1; END { say "Purchases: \n", join "\n", map sprintf("%40s (%d)", $_, $x{$_}), sort { $x{$b} <=> $x{$a} } keys %x }' /Users/*/Library/Application\ Support/KoLmafia/sessions/*201712*

While this does the 2*th of December:
perl -wlnE 'next unless /buy ([0-9]+) (.+) for \d+ each from/; $x{$2}+=$1; END { say "Purchases: \n", join "\n", map sprintf("%40s (%d)", $_, $x{$_}), sort { $x{$b} <=> $x{$a} } keys %x }' /Users/*/Library/Application\ Support/KoLmafia/sessions/*2017122*