....most grocery stores operate on a 5% average profit margin.....
But the big difference is that a large part of the inventory in a
supermarket is turned over every 72 hours.  In a past life I've 'done
the numbers' for retail operations and a big factor is 'time on the
shelf' and the cost of money.  An operation that stocks all the little,
obscure parts so that they are available across the counter virtually on
demand has a BIG chunk of change tied up in parts for a long time.   We
have a number of vendors that have made that commitment to be able to
provide us with the parts we need  to keep our 15 to 20+ year old
vehicles running.  I don't argue with their prices and just hope that
they are making enough money to be able to stay in business so I can get
what I need when I need it.  If they could turn the bulk of the items
they sell every 72 hours like a grocery store then the  markup could be
lower.  Until then I keep in mind a sign I saw on the wall in a pizza
joint many years ago  "We don't argue with the competition that sells
for less, they know what their product (and service, my addition) is worth".