What I just realized about Sam Ash
Going back to the original post, you should not be considering "Mark-up", since most retail operations work on the basis of xy% (60% in the example) of expected selling price or something similar. However that is only a trade value which they set, not for something that is supplied brand-new. More on this a bit later.
Also something to consider is what is being purchased. The gross profit maybe based on 60% (which does become 40% of the sale $) of retail for a used item, but the gross profit is still only 40% of that eventual sale, and may not even be that much after the sale, including haggling on the other end.
But consider that you are using a used item (trade-in) as a value from which to figure profit, not a new item. The cost of new inventory differs from business to business and the quantity purchased, but many businesses would be happy to clear a 20% gross profit on a new item. It is often less. I worked for a company that had a key item for sale with only a 5% gross profit, and they ended up taking a loss on each one. But the profit margin was much better on all the accessories and warranties and other "add-ons" which drove the business. Ot that putative 20% gross profit I mentioned of a new item, 15% may be figured into the ledger for general expenses, maybe leaving 5% for net profit.
And why do I mention new product? Because you didn't. Most business survive by selling new product because it brings the customers into the store. So if they can sell used product (usually less demand items, and certainly an uncertain supply chain) they can maximize their profit margin on this type of item, but can only run a business with this gross profit if they have a steady source stream or an unusually large inventory of trades from which to work.
Brick and mortar retail is a hard business, and is very fragile as is evidenced by the last couple of years. Overhead is much greater than on-line sales, but the sale is immediate and at hand.
Don