What is the average dealer mark up?

It ranges from what you would find at high volume Walmart to that you would find in a low volume, high overhead, exclusive Soho boutique (pre-pandemic).

I like my hifi vendors somewhere in the middle.

BS that markup on cables accessories, and tweaks is similar to electronics and speakers. That is where the big margins have always lived and where it is taken to the limits in "High End Audio" anywhere by those who can.

When I was in retail, we were trained to always make sure people had the accessories they needed like speaker wire because that is where the money was made.

Also teh markup on speakers was typically much higher than the markup on electronics. That was for popular brands that sold many units. With exclusive boutique brands, one can be certain the markup is higher in general because that is the only way to stay in business selling few units.

Here is an example of a quarterly P&L 
Revenue: $170,946
Cost of Goods Sold(COGS)$88,244
Gross Margin:$88702
Gross percent: 52%
Operating Expense $21,946
Operating Income:$67,106
Taxes: $9927
Depreciation: $1025
Total Expenses: $114,692
Net Profit: $56,254
Profit percent: 33%
This is a synopsis of a company that I worked for many years ago. A short example of where the profit goes. Unfortunately most people never look at a companies P&L statement and only look at the Revenue at the top.

The "high end" tweak hawkers will never disclose their margins.  People would riot.