Code:
on *:text:!bought &:#private-merch:{
  if $$2 !isnum {    .notice $nick Usage: !bought <number>  }
  else inc %bought $$2
}
on *:text:!sold &:#private-merch:{
  if $$2 !isnum {    .notice $nick Usage: !sold <number>  }
  else inc %sold $$2
}
on *:text:@bought:#private-merch:{
  .notice $nick %bought bought, %sold sold, $calc(%bought - %sold) in stock
}